The push to waterborn paints is primarily to cut down the amount of solvents introduced into the atmosphere. The biggest difference between the water and the solvents is that the solvents have a much lower evaporation point, so they will tend to evaporate more readily on their own. Granted, you still need forced ventilation for the air movement to outgas the solvents prior to flash so you don't end up with solvent pop in your catalyzed finish. Water, by contrast, will require much more air movement (and/or additional time) to realize the same amount of evaporation rate. You'll notice all the air movement devices being marketed by the automotive paint industries, these are almost a neccessity to get air into the nooks and crannies the air typically doesn't flow into, to help the water to evaporate. For home setups that are typically lacking in the air flow department, changing from solvents to water born would undoubtedly add considerable time to the process to insure evaporation is sufficient.
