I recently made the switch from vB4 to XenForo, and I have noticed that it does not allow my users to choose which user group that they'd like to have as their default. I find this rather dissapointing because some users like to support the forums, and purchase multiple subscriptions (account upgrades), and I feel that they should have the option as to what group they want to set as their default one. This is important because the paid subscriptions are mainly based on CSS mark up, such as blue sub, red sub, etc.
I don't think that they should be forced to have one specific user group based on a markup hierarchy that I set. I'd rather have them choose what group they want as their default group.