Aren't RCs usable across all Dapps?
Yep
In that case, one delegation should always cover all needs
Well depends, let's say steempress has to create a lot of accounts, we probably won't delegate enough for you to post/comment 20 times per day, probably enough for 5-10 posts to get you started. And if you don't get hive power, you may end up needing more RC, and then you could ask another dapp for an rc delegation, because steempress won't have enough rc to give you more.
I see, thanks.