Need some help with bathroom walls....

Hi everyone, Thank you in advance for your help. My question, I am going to remodel a rental's bathroom, and the person I have hired is stating we are going to use 1/4 hardiboard for the walls that surround the tub....even though the walls are currently 1/2" thick now...he states that this will allow for a 'better fit' for the tile....(I have purchased porcelain tile)...not sure I understand the reasoning, is this right? I mean, how will the seam look where the tile meets the drywall that is outside of the tub enclosure? Your thoughts? Does this sound like someone that knows what he is talking about? He also wanted to use some type of pre-mixed mastic for the tile, which also, although I have seen it in home improvement stores, don't think that is what we want to use, and I requested he use thin set... Please help me ;)