I don't understand why this is sometimes done just to find out the genetics of one leo. Won't you come up with a mess of unknowns in the process. Would it not be easier to just buy something from a trusted breeder.
Buying from a breeder who understands genetics and is trustworthy is obviously very important. However, not all breeding a gene out is to see what you have. It often times is dealing with proving out wither or not an animal is indeed heterozygous or not. This has been done frequently with the eclipse gene recently because it was abundant in Tremper albinos but had to be bred out before the eclipse could be introduced into the other albino strains. Does this create a mess...it certainly can if it is not done right, proper documentation, and resale of the "test breeding babies" is either not done or sold with clear understanding of them being pets and not breeders.
Dealing with genetics and breeding you are already playing with fire. We weaken the bloodlines by not crossing as much as we should and have created deformities and genetic hiccups and breed to produce more...so really we already made a mess. A gecko that is tremper and bell or what have you is no weaker or capable then just a tremper or just a bell. In reality it only ruins the market of selling morphs. JMHO
Here's another reason for "proving out": Sometimes a gecko is "50% het albino", which is a responsible label. Breeding to an albino is a way to prove it out without getting unknowns (as long as you're sure what type of albino it is). If it doesn't prove out, you will get all non-albinos and they will all definitely be het albino. If it does prove out, you'll get albinos as well as het albinos.