If you're running a deck which can accept a handful of dead (or just worse) draws as long as you get to specific answers, this actually makes sense.
Put simply (perhaps overly simply), the reason to run 60 cards is because you can quantify how good each card is in your deck, and this maximizes theoretical "goodness". The 61st and 62nd card - the cards that wouldn't make the cut if you cut to 60 - are (if you're building correctly) inferior cards, so if you draw one, you draw it instead of a superior card. It might just be because the ones that make the cut are the "right" answers in the current metagame and not because they're inherently stronger, but the point still holds.
In short, adding extra cards basically reduces the average quality of your draws because you'll be drawing those instead of something slightly better.
HOWEVER
That reasoning is overly simplistic. If the net gain of being able to (on average) answer more stuff because of card 61 exceeds the net loss of having slightly worse average draws in any given game, then the 61st card is worthwhile.
More intuitively, if you're at turn 5, having drawn 11 cards, and you're ok with one or two of those being dead draws in this particular game because the extra card you picked up will win you the game, then this strategy is worth it.
I don't know if this is in practice a good strategy - I assume not - because I can't think of a lot of cards I'd put in at slots 61 or 62 that would seriously help me against any particular metagame that wouldn't just be main deck, enough so to overcome the problem of an occasional extra dead draw. But the reasoning is not crazy, at least.
Another good reason to play fetches in mono-ish black decks has to do with deck construction, of course - you can fill in white or green answers to things black has problem with (e.g. dealing with a Luminarch Ascension that is about to go off via Maelstrom Pulse, whatever your deck happens to need help against, that's just an example) while only running a tiny handful of lands that can't count as swamps.
If you're not running any cards that count the number of swamps in play, that of course doesn't matter, but there are some useful options available in that department. I'm talking vampires in the abstract here, of course, and not the specific decklists people have been running in the last few days.
Put simply (perhaps overly simply), the reason to run 60 cards is because you can quantify how good each card is in your deck, and this maximizes theoretical "goodness". The 61st and 62nd card - the cards that wouldn't make the cut if you cut to 60 - are (if you're building correctly) inferior cards, so if you draw one, you draw it instead of a superior card. It might just be because the ones that make the cut are the "right" answers in the current metagame and not because they're inherently stronger, but the point still holds.
In short, adding extra cards basically reduces the average quality of your draws because you'll be drawing those instead of something slightly better.
HOWEVER
That reasoning is overly simplistic. If the net gain of being able to (on average) answer more stuff because of card 61 exceeds the net loss of having slightly worse average draws in any given game, then the 61st card is worthwhile.
More intuitively, if you're at turn 5, having drawn 11 cards, and you're ok with one or two of those being dead draws in this particular game because the extra card you picked up will win you the game, then this strategy is worth it.
I don't know if this is in practice a good strategy - I assume not - because I can't think of a lot of cards I'd put in at slots 61 or 62 that would seriously help me against any particular metagame that wouldn't just be main deck, enough so to overcome the problem of an occasional extra dead draw. But the reasoning is not crazy, at least.
If you're not running any cards that count the number of swamps in play, that of course doesn't matter, but there are some useful options available in that department. I'm talking vampires in the abstract here, of course, and not the specific decklists people have been running in the last few days.
If the question is are they a key component of a vampires deck, the answer is no.
If the question is "Are they valuable enough to be worth buying playsets of?", the answer is that totally depends on your budget.