3.5 (and presumably, 3rd) edition armor rules:

The "maximum dexterity bonus" restrictions on armor, and the armor check penalties, make for some unexpected results in the armor class of various armors. If anyone has a comment or further observation about this, please let me know.

For example, taking dexterity and armor bonuses together, the best (non-shield) armor class improvement is +9, either available from the lightest armor (and a _very_ dexterous person), or from the heaviest (and most expensive). But a +8 dex bonus is unlikely; let's take some more typical values:

For Barik, a dwarven ranger with 14 Dex (+2 bonus), he can get a +6 bonus from a chain shirt (and keep all his ranger skills, and suffer a -2 armor check penalty), or he can squeeze out a whopping +7 bonus from several (but not all) of the heavier armors, until he can get a +9 from the full plate, if he ever found any. So, the first ac bonus comes at the price of (at least) a -4 armor check penalty and his ranger skills that require light armor. The bonus up to +9 costs a -6 armor check penalty.

Or, you know, he could get _any_ old shield, and get the +1 ac bonus for another -1 on the armor check (or +2 for -2 on the armor check). Cost of a shield: slows down archery, and prevents two-handed fighting. Unless you use the shield as the off-hand weapon...

Summary: shields seem to be completely worth it for the bonus they provide at minimum cost.

For a +3 dex bonus player, the calculus is very similar: they get a +7 with a chain shirt, and the only possible improvements are +8 with a breastplate or +9 with full plate, both with the same armor check penalties as Barik. For +4 dex, only the full plate provides better ac than the chain shirt, and for +5 dex and above, the lighter armors get as good as a chain shirt, but their best possible armor class doesn't get any better with higher dexterity until a dex bonus of +8, when the padded armor is as good as full plate, with nowhere near the drawbacks.

Compare with increasing one's strength bonus, which always increases attack and damage bonuses. (Okay, there's a limit to the bonus for archery with composite bows.)

In the Players Handbook, fighters are mentioned as typically wearing heavy armor: is this because they are expected to have low dexterity? That's counterintuitive for me, but most of the medium armors only make sense if your dexterity bonus is +1 or less.