Many mainstream tans boast a subtle guide-colour. This can be great for the upmarket salon, particularly those with the more sophisticated client and/or the non-teenage demographic, because the last thing these customers want is to walk straight out of their treatment looking like... well, like they've just had a spray tan! They want a subtle glow, safe in the knowledge that their tan will be developing underneath.
For competitors, however, a subtle guide colour is neither necessary, nor particularly desireable. A strong guide olour allows for easier application, which is particularly important when performing body contouring (imagine trying to accentuate particular muscles 'blind,' with no or a very light guide colour to show where you have shaded).
Several years ago, the pervasive belief among many was "the darker the better." However, a number of scientific studies have since disproven this concept, showing that the greatest colour change occurs up to a concentration of around 10% DHA, with further changes observed until around the 14% mark. Higher than this, and you can expect to see very little additional darkening.
All of this means that for the normal, regular fake tan user, going above a 14% is of little benefit. Given that the effects of long-term DHA use are still unknown, it would seem prudent not to unnecessarily load the skin with this chemical. The 'ALARA principle' (As Low As Reasonably Possible) that we so often use in medicine is applicable here.
For the competitor, however, the story is a little different.
In the world of bodybuilding, for example, an individual will often have a short 'competition season' - with the rest of the year dedicated to working towards their next set of goals. Everything is fine tuned to present the body at its very best, and a good spray tan is an essential part of this process.
For such individuals, going that little bit darker for a special event is justified. The ends justify the means.