Actually, Parking is no longer free after 6. I don't think that's been the case for a couple years now. (I remember it was free after 6 in June 2008, but it had changed when I was down in November 2009.).
Yup... Taken from the Universal website:
Daily parking fees per vehicle:
* Regular Parking $15.00
($3.00 between 6 p.m. and 10 p.m.*)
* Preferred Parking $20.00
* RV/Bus Parking $20.00
* Valet $15.00 (under 2hrs)
* Valet $25.00 (over 2hrs)
* Valet Red Carpet $35.00
Now...as for is the side trip worth it..... honestly it really depends on what you are into, and what you might think of doing if you didn't go. Margarritaville by itself is ok, but I don't know if I'd say it was worth the trip (and parking) for just the one place....especcially since it is very popular and can be a bit difficult to get in without a long wait.
But.... they also have a nice theater at Citywalk, and several other nice clubs/bars/restaurants, so if you are doing it as more of an evening adventure, It may be a bit easier to say GO FOR IT!
You also didn't mention who "We" is. Since Disney closed PI, Citywalk is the de facto location for nightlife for us tourists. (I'm sure the local's have their own haunts/areas they like, but since I'm not familiar with them, Citywalk wins). There is a lot you can do at Citywalk if you enjoy some after-park type entertainment. If however there are kids in your group, that may not be an option for you.