1) It sets a precedent
If you give something for free, people are not grateful. They will expect more things for free in the future. If you don't charge for this add-on like functionality now, later, people will expect any other add-on to be completely free.
2) It really should be an add-on (no feature creep, please)
I don't think all and every community needs a Resource Manager. Even if it can be disabled the fact is that other people may not need it to be present, at all. By it being a separate, thing, you contribute to the feeling that XF has a really strong core, and that extra functionality can be enabled
XenForo should not be "bloatware" with a lot of features added. People complain a lot about features they really don't need. This way, they would not even get them.
3) Extra money helps XenForo
Especially for the early stages, the cash flow is really important. Do you expect a bunch of people buying licenses because there is a bundled Resource Manager? Most likely they would buy xf because a Resource Manager can be added easily.
4) Keeps people real
Software development costs money. Giving stuff away does not contribute to the fact that developers need to eat and support their family.
It is not like 1.1 would be an empty release without it. Just the awesome features here (http://xenforo.com/community/threads/development-update-may-2011.16084/#post-211750) are enough for a 1.1 release, the Resource Manager could not be there and the release would be awesome.
If you do follow this model, I have a couple of things to ask (not that you have to listen to me, of course)
1) Make the price fair. Certainly not $70, but.. maybe $20? Make it so it is not prohibitive if I just want to drive-test it for something in my forum
2) Make it for-life. I hate renewing add-ons. I wish they were covered on my license renewal. I would not care if the add-on was a little pricier if it were free to use for life.
And last ...
Please, do not make the same mistake as other forum software, that released a blog add-on and completely forgot about improving it. I once suggested that the addons should have their own team dedicated only to that add-on and not sharing time with the main forum product (or reporting to the main product maanger). Maybe XenForo is not that size, but it is sad to see the add-ons being offered as an awesome feature once and as a rotting feature 2-3 years on the run.