While it is common practice in most communities to limit perks such as avatars, ranks, and privileges to users that have been around for a healthy amount of time, the key is to find the right balance.
The theory that users will stick around longer and be incentivized by these types of rewards certainly has merit ("gamification" is the new buzz word in the industry). However, gear the rewards in such a way that makes them too difficult and you either incentivize spamming or contribute to users simply giving up and moving on.
With repect to MacRumors, the community appears to be very healthy and avatars are not as sought after as they might be on a gaming enthusiast community where representative imagery is considered to be a standard feature.
You also need to consider the possible ramifications of opening avatars to all. Abusive or inappropriate avatars can be an uphill battle - one that I have personal experience fighting in communities larger than this one. Let's just say that there are better uses of your time as an Admin/Mod than playing avatar cop all day every day.
I tend to lean toward the school of thought that avatars are something that should be granted after a fairly short period of sustained participation, but this is not my community and I am not the Commnity Manager here. At the end of the day it is their decision and we all have to live with it.
The only other thing I would say is that we shouldn't fall into the trap of thinking that only users with avatars are credible or have unique perspectives to share. We all have our areas of expertise. No registration date or post count can ever fully reflect the quality of someone's contributions.
Whew, sorry for the long-winded post. Old habits die hard when you get me on a topic like this.
