I use a cold plate it mine and it works great. The drain open thing doesn't come into affect unless you are pouring lots of beer through the the box, and the kegs are not refrigerated. I drain the water out of mine once for a three day camping excursion.
This makes a lot of sense. We only use ours for club events where were are going through 20-50 gallons in 4 hours. I think it would also matter how many taps you are planning. We have a 4-tap and a 10-tap system which both use plate chillers. I haven't prices smaller systems, but a coil system might have an advantage on a 1-2 tap system.