It's when you get it up on a crowd that's receiving all kinds of passive healing that it gets potent.
There's room to tweak, and if you show up to group PvP with intent to try to help support your team with some sort of healing, healing debuffs suck. It just becomes a question of whether to remove them altogether or tone them down.
You might want to add direct healing to that statement also.
I appreciate your viewpoint of the healing siphon and it is obvious to me, as what you like to refer to as a "pure healer" which I play, that that portion of it needs to be tweaked;
Now, even though I do have a level 50 Rogue, I tend not to play that toon in PvP as it seemed on the weak side (and after reading all these posts, realize that might be an understatement) I don't believe that is the only issue here. What I saw in the few Warfronts that I played in was a dramatic, I say again for emphasis, dramatic change in the AMOUNT of damage everyone was taking, that I had absolutely no hopes of healing through nor having time to cleanse and heal at the same time, watching people die in twos and threes so fast until I was the only one left, OR as soon as I came "out of the gate" I was jumped and pretty much three shotted. IMO this is a wee bit more than a tweak, and if it did just need a tweak I wouldn't be so vocal about it. This is an outright ***** up.
Also, as another point which you said yourself and Taugrim pointed out, "Yes, they can be cleansed, but that requires a GCD on the part of the healer, and within a few GCDs the FB or VM debuff will be back on the target again." all you have to do is spam fan and put it back up.