This shouldn't really be a question. Eclipse and the AndIRC team have came up with a 1-click-root for the Eris. Even before that, there was the Eris Master Application to help you through all the different steps.
Once rooted, you don't HAVE to go get experimental and try out all the different roms. You can use just a stock/plain rom, like Ivan's, or Senseable, which is generally a good starting point.
Having root is like...awakening your phones hidden potential. It just allows you to make the phone really how you want it to be. Honestly, I just played with my friends two Eris that aren't rooted and got frustrated.
Pros?
1.) Mobile Hot Spot/Wired Tether -> I've been using this all week, and will continue to use this as much as possible until they take away my unlimited plan. I think one of the best feelings I've ever had was sitting on the train home, on my laptop and iPod Touch (at the same time) using my 3g-hot spot from my phone.
2.) ROMs. Variations of user interfaces and designs. Custom Boot Animations, which are awesome.
3.) Most roms get rid of stock apps you don't want, like Visual VM, Amazon, ect.
4.) Most importantly...the Eris is most likely not going to get FroYo. So, if you want any chance of continued support, the AndIRC Devs are your friends.
5.) chat.andIRC.net is always there if you run into problems or questions. All are really nice guys.
6.) Overclocking. 'nuff said.