Quoted from arzoo:
Same thing with JJP POTC, they could only afford one voice; Gibbs (Kevin McNally), but he did a great job. I guess these days the A-list actors are just too expensive. Crazy how back in the 90's Williams could get the entire STTNG cast to do the callouts. As for R&M, it would have been nice to have some lines from Jerry or Summer, but not going to complain given we've got SO much content from Roiland!
LOTR set the bar very high for me. It's got iconic lines from every main character in the movie along with original callouts from multiple actors. That being said, what I've seen of R+M is significantly more impactful than the lines heard in LOTR. John Rhys-Davies/Gimli did a phenomenal job with his material and really made some iconic callouts, but at the end of the day it's Gimli yelling "Jackpot!", he's not interacting with the player in a context-specific way like we've already seen Rick/Meeseeks/etc do in R+M. Spooky did an absolutely kickass job of using Justin's talents to great effect, and it looks like this game is gonna be something truly special.