Why is it pointless? Are you suggesting energy prices will never rise again?
No, of course prices will eventually rise but I've read that many experts don't see the oil price going back up for some considerable time so there's a good opportunity for energy prices to continue to fall. Given Labour has promised to freeze prices only for two years it seems rather pointless to set the old price as the ceiling when prices are unlikely to reach that level over the next two years. If on the other hand ( and Labour haven't said which way they plan to do this freeze ) the idea is to freeze the price at the level when Labour take power then it will encourage the energy companies just to stay at the fixed level even their costs continue to fall.
The policy is much the same as the promise of increasing the minimum wage to a level it will get to anyway ..... regardless of who is in power.