A few more words on this - defense protects against physical damage & MR protects against magic damage. True damage ignores both. So Def/MR does not help against quite a few enemies (especially at end-game). But more HP will help out no matter what type of damage it is. Plus I think more skills scale with HP than Def/MR. And nothing scales with both, so no need to boost both Def & MR at the expense of HP.