For some people, voiding the warranty would be a downside. I did root, but it definitely isn't for everyone, really only for tech enthusiasts who can fix crap when it stops working. Encouraging everyone else to root just results in a thousand threads a day saying "OMGWTFBBQ I haxored my phonez and now it wont start."
The warranty is not voided if you install a stock rom back to the phone which is easy to do. Also I said "if they care capable of doing it." So I do encourage everyone who is capable to do it.
It's a pretty tiresome discussion but I'll note just for the record that there are legitimate reasons not to root even if one is "capable of doing it." If your Droid is a toy and playing with it is a hobby, rooting can be a rewarding experience.
If, on the other hand, your Droid is an essential tool for activities like, you know, your job or, you know, your life, the downside of rooting is that you tie yourself to a development path that is unsupported except by a small and shifting group of hobbyists. And when you run into issues with your phone's performance and reliability you have just one more (very large) set of differences that have to be accounted for in any diagnosis and correction.
With a stock phone, there are literally millions of other users whose droids are more or less identical to yours. With a rooted phone running a custom rom, the chances are there are perhaps hundreds or maybe dozens of others whose configuration might resemble yours. Being an alpha tester for a developer can be an enjoyable experience as long as you realize what you're letting yourself in for. Otherwise, it may well make more sense to rely upon the extensive regression testing that a stock version of the o/s involves.
In short, if you're fortunate enough to have a Droid as a toy playing with it can be fun. Treated as such, there's nothing wrong with rooting and customizing the o/s to suit your fancy. If you have priorities other than tinkering with your phone, you may find the stock version of the phone meets your needs.
