I've always liked on invision that the main things are coded by IPS like portal, gallery and nexus (although I think for most people nexus is too big, Im talkin more of something for subscriptions). Reason being is because they are more tightly integrated with the main software, and they are looked after during updates. A company will not bring out an update that will mess up one of their own products, however there is no guarentee this will not happen with a third party one and so you are waiting for updates. On top of this there is the fact that you know you have paid guarenteed support.
This said however I think thats as far as it should go. I wouldnt want every single feature you can possibly think of being part of the main system, for example build a portal and give people the hooks and libraries to build their own blocks, only providing the minimum with the base software. I wouldnt want the company concentrating on things like adding users online today, or who's viewed this topic etc ... Also wouldnt want them coding more specialised areas such as downloads, link systems, arcades and the likes that not everyone uses. I would say there is a large enough set of people to code a gallery from scratch and a portal from scratch. Personally I use a gallery and its integral to my site. I am sure there are others who find the portal integral to their site, or both.
At the moment I love the general feel of XenForo. I am under no dissillusion that there are some fundimental things missing that I would rather have such as a calender, warning system etc which are pretty much now seen as base functionality, however its version 1 and its very very good. However the more I have looked at the software the more it is just a nicety at present. There simply isn't the key functionality for me to move, which for me is the gallery and importers. Although one things for certain. I will be moving to XenForo, its just a case of when I can.