I think loyalty reward are generally a good idea, there's very little wrong with rewarding customers for shopping with you regularly, and even if reward levels are fairly low it is nice to accumulate points for something you would be spending on anyway.
With regard to the E.ON reward scheme, I'm not sure. The reward value seems very low for the amount of money an average household would be spending on gas & electricity, and doesn't even begin until you've been with them for a year. The incentive of a £10 reward to stay with E.ON when changing supplier might save more than that a month seems pointless.
If I were with E.ON anyway, and they were consistently the cheapest supplier for me, then I would prefer to receive the reward as Tesco vouchers as they could be worth up to 4 times as much, however if you don't use Tescos you might feel a bit cheated.