What is the point of PR for the London Assembly?

On the day Boris Johnson becomes master of all he surveys, it's worth pondering this point. The principal rationale for proportional representation is 'majority rule', ie. no more compulsory ID cards imposed by New Labour on the basis of 35% of the popular vote, and no more unfettered Thatcherism on 42% of the vote. But how is the London voting system supposed to achieve that objective? The key problem is that for the only substantive decision the London assembly is permitted to take - to reject or amend the Mayor's budget - a two-thirds majority is required. This means the Mayor can be elected on a minority vote (quite probable on the supplementary vote system, since in practice most second preference votes are non-transferable) and then exercise total control provided that his party has a blocking minority in the assembly, which only requires 33% of the vote. All in all, the practical effect of 'electoral reform' in London looks suspiciously similar to first-past-the-post to me.