I'm in 2 minds about this. On one hand, the longer cap allows people to budget more easily for the next 6 months. On the other hand, as you say it does mean there's a lag in how prices reflect reality (meaning both problems for suppliers if wholesale prices shoot up, and for customers as the sudden increase is higher than if it was 3 monthly).
As long as the cap comes down to reflect lower wholesale prices as quickly as it goes up then on balance I think it would probably be a good thing. Whether that actually happens on the other hand...
I believe they can't fiddle this right?