If your on dla or pip and you was to extend for the full 2 years, with the 10% rise in benefit coming in april it’s £6.50 a week. So over the 2 years £676 on top of the full allowance they’re already getting from many and any rise in year 2 could make it even more.
So your always paying more and which is why I personally would never extend and never have as I see no value there, but with the high AP on many cars and lack of choice some also don’t have any other option at all but to extend.