tim, the problem is about what usable means. To me the lowest usable level is the first ending in a 5 or 0 above the TA - whether it is separated from traffic below the TA is a different issue. Some units consider that the lowest usable level is the first that guarantees separation from traffic at an altitude.
Certainly wherever I have done approach there has always been a TV showing, amongst other things, the Transition Level. As I recall, at one unit this was the lowest FL that existed whilst at all the others it was the lowest that provided separation from altitudes. The latter is convenient and saves the controller from having to think too hard about what FL he or she can allocate but loses a level that might be available. At my last unit the TV showed both the first available FL and the FL that guaranteed separation from alts - not a difficult calculation I grant you but when it was busy and thinking time was in short supply it was very useful to have it!
We don't fly at the transition level though...
There are no rules about not using the lowest FL that exists - all the book says is that you don't cruise in the Transition Layer. From an ATC perspective there's no reason not to use the lowest FL as long as all the aeroplanes are separated.
In your example (I think you let a typo slip in second time around and I'm assuming a QNH of 1035), I agree that the T Layer is only 160 ft but I have no problem instructing an aircraft to fly at FL25.
But if we take your proposal to allocate FL30 as a minimum we only get 660ft between TA and FL30 so, in most airspaces/rules, the two levels are not separated.
PS - If I've got this last bit wrong I'll blame it on the rather good red wine that I have next to me.