I agree that the mechanic is rather cheap, but there's currently no way for many countries to go to war without taking a fairly permanent hit to stability. As Selucid, I think I need 1300g to go from +0 to +1 at this point in my game, so I pretty much am stalled out unless I use an assassin to start a war, otherwise I am just rolling in rebels.
There is no cross-marriage to get a claim on a throne that way, and I've had maybe one event in 50 years that allowed me to get a CB on someone, luckily it was Egypt.
The game's pretty dull if you are just sitting there FF'ing through decades waiting for a CB, watching births and deaths and watching 1-territory countries pull ahead of you on tech, if you take CB away from assassins there's not much to do in the game, imo.
I'd be in favor of taking away assassin CB's if there were more random events or diplomatic paths to CB's.