The bottom part of the chain stays see tension loads under normal stresses. If the opposite was true, i.e. the chain stay were normally loaded in compression, then dents like those would be a bigger concern. As an analogy, think of an aluminum beer can tipped horizontally as representing your chain stay. Now, put a big dent in the can, place one of your hands on each end of the beer can, and then push toward the middle of the beer can with each hand. This is a compression load, and the dent in the beer can actually make it easier to scrunch the can. Now, dent another beer can, and this time, place one of your hands on each end of the beer can, and try to pull the beer can apart... this is a tension load (the sort of load that the bottom half of each chain stay normally sees). Pretty hard to rip the beer can apart, plus, the dent doesn't affect the outcome in a negative way.