Well, I switch the RSP plugin all the time in 1964 and Mupen64, between 1964's in-built one, Mupen64's HLE one, Rice's modified one, and PJ64's default RSP depending on what audio plugin I'm using or game I'm running.
For example, I use Hacktarux's RSP with Azimer's HLE Audio WIP2 with Mupen64 and this combo works better than Jabo's sound + Jabo's RSP in a handful of games.
Having it selectable makes it easier for quick testing. Also, Hacktarux's RSP is faster than PJ64's default, so it's worth using for a speed boost. Hactarux's RSP improves with each Mupen64 release, and is already a great alternative.