Using the HPS for flower is not a "bad" thing just not preferrable. Everyone here would rather use MH over HPS for vegatation as someone stated above it gives off a blue spectrum wich the plant loves for vegatation. BUT you can get HPS bulbs with bluespectrum in them, now I havent tryed vegging with one yet but I can imagine it would be alright since HPS gives out more lumens then MH.