You are not supposed to be face tanking enemies as an assassin so yes I did compare the two. If once in a while you get hit by an arrow which barely hurts and that damage is negated, great. If any enemy is coming after you as an assassin, run then stealth. If anything I am worried that an increase to evasion will produce assassins that forgo stealth all together; duelist.

Usually, what happens is more like this:
You get hit by 1 arrow once in a while. It doesn't kill you but you lose stealth. Instantly 3 enemies turn around and shoot you down before you can even react.
5% is too low, 35% would be much more appropriate. It's a 1/3 chance of evading 1 attack. When you receive more attacks in a short amount of time, which happens constantly, the efficiency goes down very fast.
Edit:
After a nice conversation with Wavebend and some new info under NDA (we're both under NDA), I think it would be more appropriate to have a 1 in 4 chance (25%) on that passive. 1 in 5 chance (20%) would be a stretch but I can see it working too if the player has enough skills.
Let's not forget that the Assassin is one of the less played character. It's a shame because she's very strong in good hands, but she's so hard to play, mostly because she goes down easily with 1 single mistake, that most player stay clear of her. Giving her better survivability out of stealth would help her tremendously to lower the bar of entry. I say out of stealth because you shouldn't be hit while in stealth anyway. When you do get hit, it's either because you didn't position yourself correctly or a teammate moved the wrong way. In the former case, it's your own damn fault, learn to play. In the later case, I use my mic to explain why he/she shouldn't move that way. Most of the time, that teammate eventually understand he's not helping me but he's getting me killed instead. It's very unintuitive, because it would appear you're helping the poor Assassin by lending a hand, but in fact you're moving the enemy in the wrong direction making the Assassin's damage lower and the perfect target for annihilation... They're not doing it to be evil, I understand that, that's why I educate people when I see them do that 