I guess DHCP is the major features that makes RISC OS 5 a real upgrade for RISC OS 4.0x users. The large application memory slot feature might be a big advantage if you are running a 256 MB Risc PC and suffer from DA clamp/high bit DA problems.
I agree however that - at the moment - there is very little real, user-facing advantage for Select/RISC OS Six users. However, people actually paid for an upgrade from 4.39 to RISC OS Six despite the lack of real, user-facing advantages...
I also don't buy the QA argument. The changes people submitted for RO 5 so far are surely of similar quality than ROLs changes after the 4.39 release.
The big advantage RISC OS 5 has is of course its ultimate cheapness. You are not sure if you want to replace your trusted RISC OS 4.02 with RISC OS 5.xx? No problem, it is softloaded, and if it does not work as expected, you always have a safe fallback. Compare that to the 4.39 upgrade which some people had to rip out again because of severe problems.
I also cannot see the validity of the "burden for developers" argument. Currently, the biggest burden for developers is the rather expensive and sometimes incompatible RISC OS Six branch of the OS. A free RISC OS 5 variant for RISC OS 3.7 users would actually reduce the developer's burden.