I have a duelist and have so far found no practical use for these two abilities. My AC is better than any to hit roll unless I get really lucky in rolling, so I see no reason to give up a guaranteed attack for a possible attack in order to block an opponent's attack with an attack roll that will probably be lower than my AC anyway (does that make sense? I know the sentence construction is a little confusing). There are only four circumstances in which I see this as being useful, and both are pretty gray:
1) take TWF and do nothing but parry, hoping for an inflation in the number of attacks you would get by being able to parry and riposte a lot. Of course, in addition to TWF being against the spirit of the class as constructed in the D&D multiverse, wielding two weapons like this would negate a lot of your duelist abilities. So, why bother? At that point you're better off dropping duelist and playing something else.
2) Parrying touch attacks. This might have more utility, since a duelist's touch AC is (somewhat) lower. I have not been in this circumstance yet.
3) Parrying spells. Again, gray area. Although I don't see anything in the ability description that specifically prohibits this, it seems contrary to the spirit of the game and I really don't know if my group would go for this maneuver. Also, I've never had a spell cast directly at me.
4) Maybe this gives you two opportunities instead of one to avoid a really nasty attack: your randomly generated attack, which if that fails the attack still has to beat your AC (I note there's nothing in the rules that says the parry is instead of the AC, so it must count as additional protection). This might be useful occasionally.
I should also say that just about everything we fight is size L or larger, so I'm usually looking at a -4 on the parry. Recently I was in a battle where #4 may have applied, but since the opponent was two size categories too big I could only have parried on a 20. Since I could hit it with less than a 20, it seemed like the better strategy to take my ordinary attacks.
So my question is, do other people find uses for these abilities? It's kind of essential to the entire swashbuckler concept, but the mechanics seem to make it really difficult to actually get something out of these abilities.
<closing rant>: Why oh why can't the DnD multiverse get swashbuckling right? </rant>