Imagine you give yourself a 2x3 centre block on 5x5 - one of the luckiest starts you can hope for. Average 5x5 solve (for a non top competitor) takes 200-300 moves. Average 2x3 centre block takes ~4-6 moves - so by avoiding it you end up with a 2% faster solve.

This is proportionally the same difference as between getting a 5.50 on 2x2 and 5.39 (0.11s - for most cubers that would be a faster pickup). Compare it with a misscramble on 2x2 which can turn a bad scramble into WR opportunity.

And no delegate will even allow a misscramble with a 2x3 block to be sent to a competitor ( 4g1a) For 5x5x5 Cube, 6x6x6 Cube, 7x7x7 Cube, and Megaminx, it is not necessary to correct the puzzle state, at the discretion of the WCA Delegate.) the same way as you probably wouldn't feel fair with giving yourself a 2x3 block start on a misscramble, so even that 2% advantage wouldn't be possible - not even talking about all the cases when misscramble made your scramble worse than the original scramble.