ChrisR - It's exactly like mortgages and savings accounts - there is an initial great offer to draw you in, and then a year or two later the fixed rate ends, and you get dumped onto their 'standard' (ie. rubbish) rate. If you never change, then, as with mortgages and savings, you probably aren't on the best rate. Same with sticking with the same car/house insurance provider.
Yes, the infrastructure (pipes, cables, etc.) is all the same, but different companies have different admin overheads, production capabilities/costs, shareholder payouts, etc. so they can and do charge different amounts for the same product. Also remember that a low-usage household will benefit from lower standing charges, whereas a high-usage household benefits from lower per-unit charges, so there isn't one tarriff that's best for everyone.
As to whether it's a good system or not, it currently benefits those who shop around, rather than those who are loyal.
I've found this quite useful:
http://www.moneysavingexpert.com/cheapenergyclub
Fill in your usage and it finds the best tarriffs. It's good at spelling out all of the per-unit rates, standing charges, etc. and is independent. And it emails you if/when there's a cheaper tarriff available.