If I wanted to write an article on this topic, I would've already, so I'm keeping this concise in the interests of discussion / not boring me to tears.
In the past year, Mana Drain has slowly declined from being one of, if not
the scariest card to face in the hands of a good player to one of mere annoyance. It's taken a long time, perhaps too long, but finally people have incorporated at least some of the basic anti-drain measures (At this point I'd like to reference Cron's article about playing around Drain). In addition, decks have increasingly become proactive and the disruption reflects defending spells directly at the cheapest possible cost rather than sitting and waiting.
For example, MDG runs multiple Misdirections to win early game counter-fights over Ancestral Recall (both sides) and Gifts Ungiven. Just about every deck running significant black has begun running 3-4 Duress because it's proactive disruption and avoids the normal fights involved with trying to counter something. Fish has much more pressure on it to run cards like Misdirection or Daze as dirt-cheap ways to win counter wars.
This fact is even more apparent when you consider how many decks truly care about Mana Drain as anything more than a slight speed-bump. Combo doesn't care anymore because just about every answer they have is cheaper, Duress, Xantid Swarm, forces of their own, etc. and they have boatloads of threats. Gifts is more focused on running the opponent over than sitting back and trying to play control in most matches. Fish doesn't even care if it's cards get drained. Workshop decks are the only remaining decks that really take a major impact from Drain anymore and they've hit a major decline lately.
Simply put, has Drain become the slowest part in control decks? It's a reactive card that costs UU and more importantly, requires opportunity, to truly be effective.
Note I'm not saying we should all cut Mana Drain from decks ASAP, but I figured I may as well throw it out there.
Cron Article:
http://www.starcitygames.com/php/news/article/7063.html