I have been streaming MKVs from a PC to a Samsung LED (wired connection) for a year or so without issues, with the audio going out of the TV to an AVR via optical. Its all you need really, but I did add an HTPC for the following benefits:
- Better looking user interface with XBMC
- better control of subtitles and settings
- Iplayer (my TV doesn't support the app as I am non-UK)
- more flexibility to use different front ends and players
Disadvantages
- It does add an extra layer of complexity though, and may not be as wife or kid friendly
- takes more space, needs an extra remote, keyboard, mouse
- start up time
- random crashes, errors make it not as stable as DLNA (for me anyway)
- extra cost for the same basic function.
So its not essential at all.. but if you want additional functionality and a better looking interface, try it.