Sure you CAN play the disks on an HTPC, but:
- Playback software may not be usable with remote
- The software is expensive whereas ripping is free
- Adding rips to a media library gives you coverart, keeping track of watched/unwatched, grouping by genre etc
- I like to keep the HTPC look and feel completely non-PC like. i.e. it boots straight into WMC fullscreen and only needs a KB/mouse for maintenance which I do via remote desktop.