What I'm trying to say here is that anywhere from 3/4ths to more than 1/2 of the HP you'd get via level up are from dice rolls, not investment.

Example, you can have a max total of 56 HP at stage 6, 36 of that would be from dice rolls, whereas only 20 of that is from stat investment. Sure, you gain an advantage, but if you mess up a SINGLE dice roll, all of that effort can go to waste permanently. What I'm trying to say is, with basic stats, or at least their value, investment should at least trump dice rolls IF dice rolls are present. Dice rolls should be a nice bonus, not most of your stat.

Another thing to note... a miss or two only effects one or two turns of combat. Your max HP effects several battles, and countless turns of combat. The two aren't comparable.