Back to the original question, I think that a cancer mage is an excellent "pathetic" villain, or a good NPC to throw around as one of the mookish minions of a bigger threat. I see them as being a kind of groveling hobo king, usually-- immensely useful to an evildoer, but also pretty much unable to defend himself effectively from anyone determined to track him down.
You know. The guy who the PCs corner in an alleyway as he roots through the garbage, who rats out the big bad guy when threatened.
I see the main challenge in defeating this archetype as being FINDING him-- digging around in pubs, gathering information, patrolling a beat until you get lucky. It doesn't necessarily have to come to blows if the obstacle is searching him out; it's a nice bone to throw to the party's skill monkey.