I like the idea of the change. Dex based characters seem to have short shrift at the moment, and tossing them a bone through more parry ac would probably help even things up a bit. I feel like it both makes sense and fits from a design perspective.
I do agree, that I'd prefer tumble to also be tweaked to be better for light armor users, but if that's hardcoded then parry makes a good substitute. Also, I want to see all the +parry items we have actually matter, and it would be neat for improved parry and skill focus parry to actually matter for a dual wielder instead of being basically being rp/dump feats.
This change doesn't really affect me much, since I'm mainly playing a cleric/rogue with a shield atm, but it might prompt me to try out a monk or maybe a dex based ranger.