The problem is level availability (a growing problem that isn't going to go away anytime soon). ATC assume that as you burn off fuel you are lighter, and are then able to climb to a higher level. This allows the guy at the uneconomical level, way underneath you to climb to a more economical level (not necessarily his optimum level). Or, simply allows another aircraft onto that particular airway.
There is a lack of understanding on aircrews' part that the guy at the lower, uneconomical level is often from their own company.
Obviously, if you can't climb due to weight, or don't want to because it will put you at an uneconomical level, then just say no. Nobody in ATC is going to insist on you climbing to a level that you say you can't take. But if you can, then do it. Remember that it may be you next time at the bottom of the level pile, sitting at the uneconomical level or stuck in moderate turbulence, or stuck on the ground waiting for an oceanic clearance.
It really boils down to being aurally aware of what's going on around you. ATC are just there to facilitate the greatest number of aircraft using a particular route without undue delay.
ATCs "lack of understanding" of aircraft performance at higher levels, as you put it, is nothing of the sort. We do not have details of your load factor or weight restrictions, unless you put it on your flight plan or tell us. All we know, generally, is that a B747 doesn't fly very well above FL450 and then only at the end of a very long flight and that it can descend at M0.86 into 340kts and is generally a bit faster than a B757/B767/B737 in the cruise. And we probably know the general performance characteristics of all the other aircraft operating in our airspace, funnily enough, we do it everyday, many of us for many years.
So, to summarise. If you get a level request that you can't comply with - reject it. If you get a level request that you can comply with - think about the reason for the request, ATC don't just ask you for the hell of it. And if you are stuck on the ground, waiting for a clearance, take a moment to wonder why that is.
On the beach