In reply to Bulls Crack:
> I see it as the relationship between the 2 parameters gives the flexibility to describe the route.
Absolutely; I entirely agree with you here.
> Giving a well/resonbaly protected boulder problem route E6 based on the fact that some notional % of E6 climbers might be able to manage 6c seems nonsensical to me
That's fine too, but really I think it's better to think of how hard it is to climb overall (including that 6c move) from the floor, first go. You can work this out by comparing how hard it is to other established routes, or the notional likelihood of an E6 leader managing it. Ultimately, the latter
is actually what determines the grade -- if lots of people find it a sandbag, fail when they would reasonably expect to get up it, it gets upgraded (usually the overall grade).