I'm not a theorycraft whiz kid, I trust the Balance for that quite well. But what my normie eyes did notice is that some (maybe all but I'm not sure) encounters have "mechanics downtime" that fit perfectly with the CD reset of most buffs.
Like in O10S and its goddamn dives, yet it allowed a big burst during the Arcane Bulwark thingy where there isn't much to do (tanking, healing and mechanics wise) because most CDs would reset at that time. I kind of remember that on Chaos too.

So my point is that if you delay buffs in openers, you might hit a buff CD reset in the middle of a mechanic that's not so burst friendly. That's why "the sooner the better" is a good practice I think.