Not speaking specifically to shaman here, speaking to these changes (buff stacking) in general: this is a good thing for everybody.

If you follow raiding at all in the game currently, there is a really unfortunate trend with raiding guilds. Because they are generally comprised of min-maxers, they will "stack" the raid with exact numbers of certain classes (and usually of specific specs of those classes) to get the absolute most they can. This means that other people who happen to choose classes (or specs) that don't add to the bottom line as well as another get left out.

Then, because a certain raiding guild can put up X dps after stacking their raid, blizzard has to balance the bosses against that much dps (otherwise the 'stacked' guilds would just roll over everything). This means that ALL raiding guilds HAVE to do this stacking to be able to compete (since the bosses are tuned with that expectation), essentially mandating this behavior. Obvioiusly, this is not ideal as friends, or good players of "inferior" classes or specs, get left out of the game.

What they are trying to do here is homogenize the RAID BUFFS. Not all the classes, only those buffs that these classes bring. The upshot is, now I don't *need* 2-3 shadow priests in every raid, because there are 3 classes that can bring mana regen. Also, since it doesn't stack, I don't have to worry about 5 shadow priests being > some other 5 class. There will be one of each buff at a time, and more than one class can bring it. As long as you have one of those, you're gtg.

This means that blizzard can tune the fights assuming all of these buffs, but NOT assuming a certain raid composition since there will be many different ways to achieve it. Not only is that easier for them (meaning, hopefully, better quality tuning out of the gate, and less "gimmick" fights to try a present a challenge), but it also means it's simpler to balance segments of classes. As it exists now, certain classes, because they bring this OMGWTFBBQ buffs to raids, have to do less dps (for example) to balance this awesome utility. Now, since that utility is spread out (and can be assumed to always be present for raiding guilds), they can balance the classes based strictly on their hierarchy in whatever part of the game they fit (tanking, healing, dps).

Again, this is a good thing for all of us. You can argue that blizzard will screw it up if you want, and it's a possibility, I won't deny. But you have to give them credit for trying. It's a good plan, and I, for one, am happy to see them trying to address this very real issue.