I think Desire, Desire HD, Z, S, C, X is the lot (at least in the UK), with S, C and X being the only current ones (and it's hard to believe the S will be current for much longer). But it does indeed make searching on the original Desire a pain
It's branding. Desire was a success, so they stuck with the name. Right now it looks like the Desire name is going to outlast Sensation, and the X is a much more respectable bearer of the name than the C.
The real question in my mind is when they'll realise that the sealed nature of the One series was a mistake? I noticed this week that Sony's latest all have expandable storage, and 2 out of 3 have removable batteries again, whereas the previous Xperias (S, P, U) all had fixed batteries and storage. So will HTC reconsider, or will they continue the same mistake for a bit longer?