I would imagine a lot of people do it in view of possible future modifications, or perhaps that they think they're going to have a lot more power (and therefore require a lot more fuel) than they actually ever end up needing.

One thought might be that whoever is going to be doing the mapping may not have a base map for Yellows or Dark Blues and a 3 Bar MAP sensor whereas everyone has one for Greens and 3 Bar, so perhaps preferred by the mapper?

If your aiming for power upto 270bhp or so, and dont intend on going above, then no reason why you couldnt / shouldnt use Yellows or Dark Blues that I can see