For the annoying opponent who wants a reason beyond the rules, it would enable cheating. For example, if you stack your deck with good cards on top and your opponent cuts it without shuffling, you can cut it back to the intended state.
Suppose an instant or sorcery with Buyback (e.g. Capsize) is in my graveyard and it gains Flashback (e.g. Snapcaster Mage). If I cast it from the graveyard for the flashback cost and I pay the buyback cost, does it go into my hand or into exile when it resolves?
From what I see in the Comprehensive Rules, it seems I can choose: "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack" and "If the buyback cost was paid, put this spell into its owner’s hand instead of into that player’s graveyard as it resolves" (CR §702.33 and §702.26 respectively) sound like they both replace the normal zone change from stack to graveyard.
According to §614.15 "self-replacement" effects take precedence over ordinary replacement effects, but these two replacements should be of the same kind.
The IKEA Helmer drawer unit is alive and well: http://www.ikea.com/us/en/catalog/products/40107872/
Recommendation: don't assemble the wheels to allow staking them two or three high, as I've seen in a couple LGSs.
Note that Watcher in the Web can normally block eight creatures, not seven (seven additional ones plus the implicit default one), and this ability doesn't depend on its power and toughness or the power and toughness of the creatures it blocks.
It's a different case: multiple archdruids are identical, choosing between them is meaningless, and when their order suddenly matters later in the game any player can simply choose or target the ones with the desired position in the timestamp order.
but it is not technically destroyed so would it come back at the end of a turn.
This is the part that is pure fantasy. Nothing "comes back" from the graveyard unless a card says so. Distinguishing destruction from other ways to die (like sacrificing or the SBA that checks for 0 toughness) matters only for cards that trigger specifically on destruction or count/track destroyed permanents.
The ability triggers unconditionally (it doesn't say "if Nettle Drone is tapped..." or the like) and when it resolves it tries to untap Nettle Drone. It will succeed if it's tapped when the trigger resolves.
What is legal, is to either cast your Drain Power during a phase that a player might not be able to cast the spells or use the mana (during their upkeep or draw step), or in response to a spell (since they won't be able to cast another creature or sorcery while a spell is on the stack).
In practice, only a few spells and abilities can be played at instant speed but only during specific phases or steps; creatures, enchantments, artifacts, sorceries, planeswalkers without flash and "sorcery speed" activated abilities can never be used to answer Drain Power (or anything else), and almost everything else can be used unconditionally.
Doubling Season replaces the number of tokens in the text of the ability that puts them on the battlefield, it doesn't copy tokens like Parallel Evolution.
Me (player A) attacks unprotected Player B with Bloodcrazed Neonate. Player C allows me to hit player B, but just when I'm about putting the counter he uses an instant -1/-1 to kill my Bloodcrazed Neonate before it gets the counter. He explains, the +1/+1 counter goes to the stack, but he doesn't let it resolve, and puts on top of the stack the lethal damage. Is that right?
Pay attention to vocabulary. Getting -1/-1 with toughness 1 is informally "lethal" because it kills your Bloodcrazed Neonate before it gets a +1/+1 counter, but it isn't damage, nor lethal damage (an even more specific technical term, used for example in the rules for trample): it is a power and toughness adjustment that's governed by completely different rules. No lifelink, no deathtouch, no damage prevention, a separate state-based action with a different condition (0 toughness vs damage equal to toughness or greater) to actually kill creatures, no triggers on dealt damage, and so on. It's a fairly big deal.
Practical consideration: with Sphinx's Tutelage triggers off an effect that includes drawing and something involving a choice (e.g. choosing what to discard with Artificer's Epiphany), milling takes place after making the choice; the Sphinx player must decide without knowing the not-yet-milled cards.
If the victim player wants to take a shortcut, he should count the minimum number of milled cards without revealing them.
From what I see in the Comprehensive Rules, it seems I can choose: "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack" and "If the buyback cost was paid, put this spell into its owner’s hand instead of into that player’s graveyard as it resolves" (CR §702.33 and §702.26 respectively) sound like they both replace the normal zone change from stack to graveyard.
According to §614.15 "self-replacement" effects take precedence over ordinary replacement effects, but these two replacements should be of the same kind.
Recommendation: don't assemble the wheels to allow staking them two or three high, as I've seen in a couple LGSs.
It's a different case: multiple archdruids are identical, choosing between them is meaningless, and when their order suddenly matters later in the game any player can simply choose or target the ones with the desired position in the timestamp order.
This is the part that is pure fantasy. Nothing "comes back" from the graveyard unless a card says so. Distinguishing destruction from other ways to die (like sacrificing or the SBA that checks for 0 toughness) matters only for cards that trigger specifically on destruction or count/track destroyed permanents.
Pay attention to vocabulary. Getting -1/-1 with toughness 1 is informally "lethal" because it kills your Bloodcrazed Neonate before it gets a +1/+1 counter, but it isn't damage, nor lethal damage (an even more specific technical term, used for example in the rules for trample): it is a power and toughness adjustment that's governed by completely different rules. No lifelink, no deathtouch, no damage prevention, a separate state-based action with a different condition (0 toughness vs damage equal to toughness or greater) to actually kill creatures, no triggers on dealt damage, and so on. It's a fairly big deal.
If the victim player wants to take a shortcut, he should count the minimum number of milled cards without revealing them.