I have a Windows box under my TV that runs Plex. It also runs a web browser, which means it's incredibly simple to look something up on Google Maps, order some food from Seamless, etc. - not to mention be able to video chat on Skype and play video on any web site out there, not just ones that have apps for platform X.
If all you want to do is watch videos from Plex then no, there's no point changing hardware. But you'd be surprised how much you end up doing through the TV when you can.
If you frequently sit in front of your HTPC-enabled TV with a laptop, connecting the two together using http://synergy-project.org/ is amazing, UX-wise. You don't ever have to pick up a remote, or try to get Bluetooth peripherals to work over an 8' distance; you just flick your mouse up "past" the top of your laptop screen, do a few things on your TV, and flick back.
It's a bit of the feeling of having a second (big) monitor hooked up to your computer (or using Apple's AirPlay "extend display to this monitor" mode in OSX) but having the two be separate computers is actually a good thing, frequently:
• I don't have to worry about having something CPU-demanding (like a streaming Flash player in Chrome) running on the TV while trying to compile on my work laptop; neither one will cause choppiness in the other.
• I run all my torrents on the HTPC (because that's where I want to play the videos from anyway) and have QoS enabled on my router, so my laptop's network connection is never clogged.
There's also the benefit of the pixels of a TV being far-enough away that you can get away with a lot of scaling. Presuming a 60" 1080p LCD at an 8' distance, turning on Windows' "Extra Large" DPI Scaling, or force-enabling OSX's HiDPI mode, looks really good (e.g. https://dl.dropboxusercontent.com/u/47375701/ss20151022-1108...).
You only get 960x540 effective pixels from a 200%-DPI-scaled 1080p TV, so some things might not fit on the screen and will necessitate temporarily toggling back to "regular" 1080p. Also, you'll probably want to full-screen most apps, to get something like this (http://i.imgur.com/YDN6VuZ.png). It's a lot like using a netbook.
It's pretty great for reading long articles in particular; I find there's less eye-strain than looking at my laptop—maybe because the glyphs are so clear, and maybe also because I'm focusing on something farther than 2' away.
It's not fancy, but it does the job well - barring some signal issues that I think must be specific to my apartment, as I haven't seen anyone else complain of them.
I use a wireless mouse that sits on my coffee table. I also have a wireless keyboard that I keep on the shelf underneath the coffee table, but most of the time I don't need it.
If all you want to do is watch videos from Plex then no, there's no point changing hardware. But you'd be surprised how much you end up doing through the TV when you can.