Protectthehornet, I can see the merit of using a radio altimeter if it is working, but is it necessarily wise to assume it will?
I'm not suggesting that in the case of flight 1549 allowing sufficient excess speed to flare would have been wrong - with hindsight it might have reduced damage and injuries - but as a more general observation, expecting an accurate flare might be unrealistic, and if a plane is to be expected to survive a ditching, a flare should not be assumed.
It can't be realistic to assume everything the pilots do in such circumstances is optimal, instead one should assume they are competent, and do what would be reasonably expected. I'd say that those on flight 1549 far exceeded this...