I see that it was set to ".8" but that only seemed to be used in printing out the version number. I didn't see uses of it elsewhere, did I miss one?
That's the purpose of the field. Whenever we have a four-dotted release 0.3.20.1, we set pszSubVer. It is part of the overall program version. It is separate AFAICT because satoshi needed an extra field, and the dotted-triple 0.3.20 is already part of a fixed encoding scheme that sets converts X.Y.Z to a single integer number.
Presumably you're aware of the network protocol
so presumably you can see how it is not feasible to add a fourth qualifier to the encoded version.
So, I disagree with your change, it stomps on a field bitcoin has used in the past six months.
We can change this if/when the client version becomes separate from the protocol version. cdecker proposed this in https://github.com/bitcoin/bitcoin/pull/63
His implementation was not desirable, but I think the genesis of the idea was sound.