There are two valid arguments against kickstands.

1- they're inherently unstable, and bikes are prone to falling over. This is especially an issue with derailleur bikes, which are most likely to fall onto their right sides, possibly bending the derailleur and/or the hanger.

2- the common kickstand mounting systems are prone to crushing the chainstays on lightweight frames of better road and hybrids. This doesn't have to be, but experienced shop mechanics will confirm that it's all too common.

Other reasons include things like needing to put the bike against something anyway in order to lock it, added weight, that they get loose and become an annoyance, and of course that they're "uncool".

So, those are the reasons not using one, and can be weighed against whatever anyone's reasons are for using one, and a decision made, which is nobody's business but the owner.