Even though it sometimes affects me, I do not think temp HP should be limited unless temp Att & temp Def is limited as well.
Some ninja develop stronger attacks, sooner than others (a reason for leaving temp attack in PvP).
Some ninja have better defenses (a reason for leaving temp def).
Some ninja have more stamina (ability to outlast, reason for leaving temp HP).
I get attacked (and killed) occassionally by those using temp HP, but primarily, it is those using temp Attack & Defense. But of course, I am not at as high a level as many of those suggesting the limitation. Just my two cents.