The entire community coalescing around a single engine is ultimately a good thing; there's still room for the maverick outliers, but so long as you do-what-Quakespasm-does you'll get support for pretty much anything released.

The obvious con is that the quirks and idiosyncracies of that single engine become enshrined as part of a standard. That can cause harm - e.g. FitzQuake and earlier versions of Quakespasm had very poor performance with dynamic lights. That can hold back content authors from using dynamic lights, and at one point led to the ridiculous situation where disabling multitexture was touted as a performance tweak.

Fortunately Quakespasm these days is really rather good.