In all fairness, it is pretty hilarious watching some replays of people failing on a trapped TH.
I've done it a few times for kicks, even watched a person expend all rage/ drags, and probably a 300K+ army just to get my TH.
They try to raid the rest of my base, but are usually unsuccessful, which is even more amusing. If they zap that's fine. Spend 60K+ on zaps to maybe get 1,200 which my max lvl drills more than make up for. Also, enjoy being 100% revenged in return if you zap ;).
...But agreed on the basic premise.
TH trapping is basically a troll move. By trapping it you are removing some key defenses that would otherwise be protecting your loot better, just to attempt to spoil someone's raid. It also can cost the trapper quite abit of resources if they are using traps/bombs that will need to be rearmed after every raid.
An exposed TH is usually taken regardless of the defenses/traps surrounding it so you still dont' win many defenses.
What it does do is ♥♥♥♥ people off though, which is exactly what the trapper is intending to do. In that regard, it is successful.

