Honestly, I found out how siphon is on an ally is just last night. The biggest sell of it is the 33% revive. I was rezzing people every 8 seconds, instant cast, at range, even though they were not in my party/quad (made it harder), and I was not built for support at all. It is crazy good at helping downed characters (Looking at you, ele players), and can imagine it being HUGE in raids and strikes. It gives an extra life to that little kittenup that no-one else can rescue asides necro downed tp (which I think causes deaths as much as it actually helps).