well, i can only share my very experience.

2 weeks ago i got my fixed, i decided to run spd's until i got used to the whole fixed thing.
300 miles later i thought, ok, lets change over, so i installed Look Keo's as per my other road bikes.

Yes, there was a price to pay in having to adjust yet again, but the more solid, locked in feel definately helps my pedalling and I'm getting more drive too.

I used my spd's on fairly stiff shoes, but i have carbon soled shoes now with the keo cleats.

Not for everyone maybe, but for me, it makes sense to have continuity across my mounts.