But there's already a regular mass relay orbiting Pluto--it was frozen over until it became an ice ball humanity named Charon. Seems silly to put two mass relays in such close proximity to one another. Then again, I tried to argue the same thing about the Omega-4 Relay being in the same system as another relay and being made to look so different and unique from every other relay, which would only make people more curious about where it goes, rather than less curious, but nobody agreed with me on that one.
I don't see the Reapers putting another huge relay like the Citadel, especially one that's meant to be a backup in the event of the Citadel failing, in a star system containing a habitable planet that might spawn a sentient, spacefaring species one day. It'd make more sense for them to build it in another defensible location like the Widow Nebula, where they built the Citadel, in some corner of the galaxy where organics aren't likely to find it.