Quote from socketto »it can be very relevant and a lot of times you see this done if the card has a narrow use. cards like negate, celestial purge, or creatures with a very specific use tend to occupy this type of slot.
Negate is not narrow at all!
That being said, i tend to believe it to be the other way around. Sideboards with a lot of 1of cards tend to overlap a lot, so they can be used in a veriety of configurations to suit different matchups. Therefore they have a wider application.
Narrow answer cards are those you absolutely need in specific matchupss, usually the bad ones. Take Fulminator Mage in earlier BGx midrange VS Tron for example. Other exaples are Rest in Peace and Stony Silence in white based decks vs graveyard based strategies or artifact based combo. These usually are pretty narrow but powerfull "hoser" effects you really want or absolutely need to draw in order to win.
So by this standard, overlapping side board cards can be used as a variety of 1ofs whereas the narrow, must have answers usually show up in multiples.