Diablo 3 has a new system in place called Nephalem Valor Buff which only comes into play at maximum level (60). This gives the player a bonus to magic find and gold find each time the player defeats a Unique monster or set of Champions. Right now it looks like this buff will stack, becoming larger and larger with each group killed to some maximum. Defeating a boss while under this buff will also grant extra drops. The trick is that the buff goes away as soon as you swap out a skill for another one. The design is to allow players to swap to new specs any time they want but to discourage them from swapping constantly based on what monsters they expect to face. The other effect of this change is to make it optimal to play in a single game for an extended time. Rather than just running the same boss over and over again every two minutes we will probably do much better clearing whole areas or acts in a single sitting which I think will end up being more fun than repeating a single short section over and over.
I think this is a somewhat kludgy way to address the problem of people respeccing right before fighting bosses. I can't see swapping specs to fight normal monsters being useful since you won't know ahead of time what the next pack will be nor what sort of mods they will have. Bosses, on the other hand, are always the same so their may well be specific builds that are extremely powerful against them. Without any other incentive I can easily see players feeling they need to swap out their specs against each boss to maximize their effectiveness; that is, of course, assuming the bosses are really hard, which I think they will be on Inferno difficulty. In essence Blizzard is trying to save the players from themselves by providing an incentive to not swap specs during a play session.
There are some other things they could do to prevent the issues of boss respecs like making sure that bosses respond appropriately to stuns, fears, etc. so they can't be permanently locked down. This is probably really tricky though because if stuns and fears are useless against bosses then players are really incentivized to dump them and if the boss isn't immune then the boss just keels over dead. Making bosses immune to crowd control effects but having lots of summoned mobs that aren't immune would provide a happy medium but that isn't always going to be an option either. Just like in WOW it is extremely difficult to design all fights so that all abilities are useful but not overpowered.
Blizzard could have just as easily have made this a negative instead of a positive thing but it would almost certainly have been badly received. Instead of having a stacking bonus they could have simply put all skills on a 1 minute cooldown after changing them around to incentivize people to stick to one build within a play session but this would get people really bitter I expect. All in all I don't love this system but if Blizzard really feels it is important to prevent people from swapping specs and games constantly I think it will do the trick. They seem to have a really clear vision of how they want games and grouping to go (Not at all like Diablo 2!) and they are willing to do some unorthodox things to make that happen.