ChrisD: because the freeze plug types typically don't offer the wattage you can get with a tank style heater.
PeterB: sounds right.
Coyote: go for it! I thought I was pushing boundaries going with 800w on a 1L, but why not? Also you hit the nail on the head: the faster it works, the more likely you'll use it for "spontaneous" trips.
And now the fun part: I just realized mine must be installed wrong. I was sure my instructions told me to mount mine horizontally (outlet), but bbjsw10,
and now Mullet, both installed theirs with the outlet pointing UP, which makes far more sense considering these are supposed to rely on convection for circulating the coolant. (I realize the PDF you linked to points up.)
I checked to see if I still had my documentation, but I don't.
So I'm going to have to have another look at mine. Supposed to be getting a warmer spell this week, so I may turn it 90 degrees and see what happens.