They weren't all dodgy (although please feel free to think the worst of your rellies if you want to!), but they were quick, cheap and convenient. Ironically, the records often give more detail than you get in ordinary parish church marriages - specifically, the groom's occupation.
The whole series of Fleet Registers in RG7 at TNA is currently being digitized and indexed by BMDregisters, but so far they haven't announced a likely release date. Frankly, I'm not surprised because this is a very difficult series to deal with, with many duplicated entries, and at least one register that has been proved to be a forgery. Apart from that, some of the films are unreadable so they will have to re-scan the originals.
Mean_genie