Diablo only gets bugfixes (often, not even security fixes).

Look at it from this angle: feature enhancement should not be applied to Stable. Its for Testing. Diablo is Stable. Fremantle is Testing. Mer is Testing (but different distribution).

It is very difficult (and expensive) to test features like this in order to be good enough for release. What if there is a chance of 1/10000 a bug gets triggered crashing the X server? Many Diablo users would not like that. Therefore, those who want to live on the edge can use Mer. But Nokia assumes the user only uses the official Nokia repositories, and uses the tablet for mission critical work. They have to; its their reputation. Same for SSU. These cannot take into account deep changes. So, what you want to do is protect users who do not have the ability to understand advanced changes to be able to apply advanced changes. You can do this by making it a bit harder to apply the advanced changes, and informing them of the side effects (but not impossible; that implies a vendor lock-in). For example, some 'hacks' apply text settings in /usr but this is potentially dangerous causing regressions.

So, in other words: your point is completely valid but not for the official Nokia distribution; its for Fremantle or a 3rd party distribution such as Mer.