Just curious if anyone who plays a thief character actually goes without using any plate armor, or if doing so would totally ruin any chances of survival at higher levels.

Played one who only wore chain and he did fine. Plate armor is nice, but not an absolute necessity. Besides, it weighs a lot.

My character is a custom assassin and he wears the best armor he can find, even plate. :shrug:

My Acrobat is restricted to leather and I don't have any problem on higher levels. The advantage of playing a stealth character is that you can evade the hits better with your Dodge skill and Agility attributes. This replaces the need for heavy armor.

My thief is restrained to leather, and even then I rarely ware more than the gloves and greaves because the other pieces overlap on top of her regular clothing.

Not a problem really. If anything, magic is the big killer, not melee attacks.

Don't wear plate mail if you're an assassin, it's nonsensical and you might as well take 30 HP/level and be a high elf with weakness to paralysis.

Leather only is perfectly fine for completing the game. The trick is to invest some time/money in the dodging skill. Make it a major or minor skill though, you're gonna want to focus your primaries on Critical Strike, Backstabbing and Stealth (the beth default assassin has those as primaries and by gum it works). Dodging is fairly difficult to train on it's own, since basically you have to get beat on by critters to train the skill, but a certain guild for assassins is perfectly willing to provide training for those inclined.