Blizzard to Paladins:

[MongoJerry]

It's not hard to remove a disease.  Most people won't know, but anyone with half a brain will look at the debuff icon at some point, see Disease, and dispel it.  Especially if that debuff harms your ally while healing the enemy priest.

Small party fights generally have at least one class capable of dispelling diseases, in my experience  <_<

And in my experience of actually using the spell, no. Or at least, I know when to use it and when not to so that it doesn't get dispelled. I said that I use it in 1v1 to 3v3 fights. In those cases, either there is often not someone there capable of dispelling diseases or the person who is capable of it is too busy doing other things to notice.

The most common situation I use it in, though, are 1v1 fights versus rogues, warriors, hunters, and mages. Rogues and warriors in particular love to challenge priests (it's nice to be geared up to fight them and turn the tables on them), and having an extra damaging and healing dot in those kinds of fights is very useful, whether I'm getting ganked while out farming or have a 1v1 to 3v3 matchup in CTF or want to throw some extra damage onto a flag carrier as he or she is leaving our flag room. Small party PvP happens a lot and I use Devouring Plague so often that it's on my front hot bar. And I can again tell you that in practical situations where things are moving fast, no, even when there is a dispeller around, Devouring Plague often does not get dispelled -- whether the person involved doesn't realize that it's a disease instead of a curse or whether it's because the potential dispeller is too busy doing other things, I don't know. But, no, in small party fights, it often doesn't get dispelled, even when my SW:Pain's do. I frequently laugh about it.

I really don't know why you're playing theorycraft with me on this. I'm telling you from over 10 months of practical playing experience as an undead priestess on a PvP server that Devouring Plague is an extremely useful spell that has frequently meant the difference in PvP fights that I've had (particularly in solo fights against rogues, warriors, and hunters). Your attempt to say that for some reason it shouldn't would be like me asking, "Why do you use Mortal Strike? You can just heal through that debuff!"