Parry is very, very useful if:
1. You fight a very small number of enemies (one or two).
2. You have high dex.
3. You have picked Skill Focus (Parry) and Improved Parry.
4. You have items that gives bonus to parry.
Parrying everything except enemies rolling 20-ish is quite nice, in my opinion.
Most people tend to prefer expertise as it's better for 95% of the encounters (it's very rare that you only fight a single monster, after all!). Parry is extremly useful in duels (1 vs 1) though.
