You are paying a lot for their utility, granted, but I usually have them last a good long while (unless there are a lot of wave serpents on the board, but then the Maulerfiends wouldn't last then, either).
I use the Defiler as an area-denial unit. You park it somewhere near a few objectives and have it blast away. If anybody gets too close, you charge them. I've found it works quite well, myself. I *could* get a single purpose unit for cheaper, but it wouldn't fulfill the job as well.