Paint is not all the same by a long way its like anything you get what you pay for and the quality between cheap shit and good products is vast. One of the main issues with not painting in a booth apart from letting all your neighbours breathe isocynate is that its not a controlled enviroment with regards mainly to damp/moisture and dust to a lesser extent, if there is any damp you will get microblisters/blooming issues to name a few, were not just talking about how good it looks on the day its painted its what is gunna look like in years to come. Also i can almost guarantee that most people dont have a sufficent water trap or dryer on their compressor as an at home painter to keep moisture and any other nasty stuff out your air, this will also cause issues.

Another good point made by another chap, why is it that any dodgy back street mechanic can charge £45+ an hour but people struggle to get £30 in a bodyshop......