You sign up for eliminating of your -5.5 diopters of myopia and you end up with, e.g. -3.25 diopters of myopia. Or, even worse, you end up being farsighted with, say, a correctio of +1.25 diopters of hyperopia. The first scenario, undercorrection, is not too bad because you have reduced your nearsightedness and you can still be re-treated at a later date (LASIK - 3 weeks).