Not trying to start another task killer debate, but my suggestion would be to try your phone before you decide to add one. If the phone runs alright then go with it. If it starts to give you problems then take things from there. No real need to install something if you don't need it.
As for other apps, that is really subjective. What I may find to be the most useful app in the world, you may find worthless. Just for example, I doubt you would be all that interested in an app to track menstrual cycles. While I and he other ladies may love it.
The best way to find good apps is to decide what you want to do and then just search the market. These threads may give you some ideas though.
http://androidforums.com/htc-droid-eris/44587-list-your-apps.html
http://androidforums.com/htc-droid-eris/24900-list-must-have-apps-please-add.html
You can also search through
Androlib.com to see apps with reviews, screenshots and some videos.