Good question. I'm going looking for some myself to help a friend.
His house that is only 7 years old has one furnace in the attic.
When it got down to zero here, the condensate trap froze, water
backed up, shutting the furnace down. It also cracked the fitting,
so it's leaking into the pan under the furnace now. The condensate
trap is under the horizontal furnace and uninsulated. The idiots
who installed it put insulation on the drain pipe that runs 15 ft in
the attic, but none around the approximately 5x7 plastic condensate
trap itself.
Looking at the Lenox install instructions, they say that any install
where the temps can drop below freezing, the plastic condensate trap
and the drain line should be wrapped with heating tape. So, apparently
there has to be tape suitable for empty drain pipe use. I'm amazed
this passed inspection.
Quick google shows there are tapes rated for plastic drain pipes:
http://www.chromalox.com/productcatalog/Heat+Trace/Light+Commercial+Cables/Thermwire+Pipe+Freeze+Protection+Cable/product-details.aspx?p=316
I would suspect the issue is that there is some concern that
without water in a plastic pipe, if you wrap a lot of tape around
it, put a lot of insulation over it, it might get hot enough to
melt it. Seems unlikely to me, but in may be CYA on the part of
some manufacturers. I would be though that a lot of people are
just buying whatever is on the shelf a HD and using it like you
intend.