Blood Pact is not a 4 stam buff, but rather a 42 stamina buff, making it very much worthwhile in the tank group. The tanks should not be buff capped when having it. That being said the boss should have room for one corruption debuff, although SM/Ruin starts to really fall behind DS/Ruin in damage by Naxx.
that was a reply, improved bloodpact is 54 stam vs zanza 50 stam. If you have a spriest and a thunderfury in the raid there is zero room for a corruption. I already give up a druid and hunter most nights for buff cap space to allow for hots.
It’s entirely ok for a warlock to have corruption on the boss. I’m SM ruin and I’ve spent 1 gold on respecs since level 1. I put my curse on the boss, pop ZHC and use corruption and shadow bolt and nuke until I need to life tap, then i put up a new corruption and keep going. I use my imp for a pet during raids and thats totally ok with me. Because hardly anyone ever asks for healthstones and I don’t have to sac my pet every few minutes I end up leaving most raids with more shards than I had when I started.