What could get fixed during maintenance? Lots. Its been shown before that the staff involved can make numerous changes when the servers are brought down during the maintenance timeframe. They simply choose not to fix things during this timeframe, probably for the same reasons lots of other stuff goes unaddressed around here.
Now im not saying they can fix everything... but more can be done than resetting demonstration battle & the bloody battlefield.
On top of them, let's not forget that resetting the server tends to stabilise a lot of things on the server.
A maintenance does not need to fix bugs. Usually it is there to clear out the caches and ensure that things that are not supposed to happen ... don't happen.
Edit: Fixing small bugs isn't entirely easy (especially when it might be difficult to find where they are).
Runewaker do not have the resources larger companies do when it comes to bug fixing.