I see little or no sonic value in buying mono records, but there is likely some nostalgic value for those who listened to them in mono when they first came out.
Having said that, there is possibly (but doubtful) a sonic improvement if the music was originally recorded and mixed as mono and was subsequently pressed onto a mono pressing and then played with a mono phono cartridge. This is a long shot, but integrity of the signal path is paramount and could potentially trump the other benefits of stereo recordings. To realize this potential improvement everything in the front end of the signal path would need to be period specific (i.e from the days of mono so early 60s or earlier), holy PITA!
Such re-releases will likely be cut for stereo phono cartridges (so yes, could be played fine and no need to swap out the cartridge) and therefore the integrity of signal path has been lost somewhere down the road anyway. So needing to swap out cartridges would only be necessary when playing original mono pressings, not re-releases.