I think I'm a little confused by you're reference of a RP 'party'. Are you talking about a group of RP'ers who RP while getting EXP?
In regards to normal RP scenarios, good RP can be had in a MMO whether you are in a set group or not. I can see the unique fun to be had in having a small static RP group, but on the other hand, it could also foster an exclusion animosity in a LS. In my own personal preference, I tend to avoid static groups like this because I don't want to feel like I'm leaving any of my other LS mates out of the specific fun to be had in a exclusive group.
In regards to normal RP scenarios, good RP can be had in a MMO whether you are in a set group or not. I can see the unique fun to be had in having a small static RP group, but on the other hand, it could also foster an exclusion animosity in a LS. In my own personal preference, I tend to avoid static groups like this because I don't want to feel like I'm leaving any of my other LS mates out of the specific fun to be had in a exclusive group.