I obviously don't know who designed the current iteration of assassins, but I've played both assassins and thieves. The latter extensively.

I've said before that it feels like assassins were designed by someone who wanted to play an assassin and thieves were designed by someone who wanted to play a shaman.

Of course, Since their initial designs, I think assassins have gotten more powerful and thieves have been nerfed.

I don't think assassins would be at all under-powered if you just did away with trance all together.