disclaimer:
I'm not a KP developer, so what I'm about to say may not be correct.

Only basing KP on a newer kernel version wouldn't change a lot for the end user. In fact I think he wouldn't notice it at all. But being able to move to a recent kernel would change 2 things:
1. KP developers wouldn't have to backport new features or security patches to 2.6.28. Instead they could just use the vanilla kernel or some distribution kernel and had lots of spare time for other cool things because they wouldn't have to do things again which was already done by others.
2. For KP to be upgradeable to an arbitrary newer kernel version would require the binary blobs to be open source. That means a skilled KP developer could examine and judge all the possible consequences a kernel upgrade would bring. As a result the whole free userland of Maemo could be upgraded to newer versions bit by bit and the proprietary elements could be replaced with potentially better open source equivalents because with a completely open kernel we had access to all the interfaces and there were no more blocking dependencies.

I believe that a completely free Maemo would be part of the Debian distribution by now (or at least an unofficial project), minimizing the effort necessary to upgrade the system. Easy Debian would be needless because everything Easy Debian does would be possible in Maemo natively. It would even save resources (mostly RAM). Any project that deals with making some software work on a Maemo device would save a lot of work because (assuming they are already part of Debian) all they had to do would be to check if their GUI is useable on the device.

I'm not a fan of Android at all, but for those who think differently: If Maemo was completely open source porting Android to Maemo devices (including phone support) would be much easier.