If sin gets knocked out of platform in high he must go a distance to get back, while marauders can just jump back (if target is not in cover, but even then you usually have to wait a bit out of LoS for gunslinger to come out).
It's a paper - rock - scissors thing. Still maras don't have a 2 sec 150% speed boost, mara can't go stealth, not a proper one.
Next, you can't build proper melee train team with sin assisting because of usability to change focus to distant target - almost no other dps suffering from that. That alone can be a valid reason not to include assassin's in arena teams where you have to coordinate focus fire.
As i said - BW won't balance pvp around the hardcore players, at least not in way they want. This game will be always balance around the pug with small factor from new ranked games. This sucks, but buffing sins/shadows in the way you want it would crippled the pug games coz sins would be new FoTM and this would be a big one.
Second - BW said they will do some changes, probably nerf/buff mix bag - because the game is in the pretty balance state not a perfect one, but also not in the bad one. And this will be made after they get how arenas plays out on live servers.
So i don't understand what's the whole commotion. There will be changes, in the 2-3 months max, when the 2.5 patch hits.
Next, force camouflage just enough for marauders to shake off focus and get healer a chance to heal him up. And sins force cloak usually busted a lot more frequently due to dots good opponents use on them - unless you fast enough to use blackout and it's not on CD.
You can't be healed while in the stealth. Maras have 4 sec one after which they are vulnerable. Sins (if there isn't any DoT on them) can stay stealth. Even if they could be healed it wouldn't be a big numbers - because it would be only 4 secs healing and no class can heal to full in 4 secs.