I’m an ‘old hippie’ (66 in October). I know a lot of folks use ‘rattle cans’ for all manner of painting. Me? I hate that they pretty much ALL use VOCs as their propellants. It’s why many of those paints at a local store (like Wal-Mart) are locked behind a rack. That whole ‘sniffing glue and huffing paint’ concept hails back to my mid 1960’s entry into plastic model kits.
More importantly, to my view anyways, is that the level of ‘control’ you have with a spray can depends on distance from target. If you’re doing something at 1/24th scale or better, it’s not as much of a concern. If you’re just priming a model (or a lot of models) then, again, it’s about decent coverage at a reasonable distance.
My general dislike of using spray cans goes to pretty much all those things: VOCs (bad for environment, you and sometimes your model surfaces), hard to control the level of spray, overall, requires a fairly good, open space, lots of ventilation (and/or a mask) and if you don’t use it all now, hmm, you might as well forget about it later.
They ARE generally cheaper than the tiny pot/bottles that the model industry seems to prefer (I still have an old Testors bottle with a 15C stamp on it). In my stash of paints from yore, I have testors and Tamiya enamel paints. I still have some more than 25 years old that are still good.
I have now also claimed some newer acrylic paints, though have yet to test them out on anything – I’m really just getting started back into the hobby after more than those 25 years of inactivity. I snagged a decent airbrush setup (compressor, two airbrushes and one spray gun, hose and connectors) for $65 US. I’m just getting to the point of prepping up my own DIY spray booth for all that.
Spray cans really can lay down some paint and if you’re practiced enough, you can do a pretty standout job of application. Personally, that’s too much strain on my finger (the one holding the nozzle button) and the cans are not what I would call hand friendly when it comes to the level of fine detail I prefer.
As to a ‘rule’ I haven’t heard of any. I’ve been a ‘low budget’ and DIY guy since I was turning in Coke Bottles for 3C and 5C for the big ones and beyond. So finding a way to do what it is you are intending has been my ‘normal’ for all this time. If spray cans work for you, more power to you, I say. This hobby has only one real rule – do what’s right for you.
I prefer making dioramas and usually worked at 1/72nd and 1/35th scales. I use the 1/72nd for my ‘grand scenes’ and the 1/35th for smaller, more personal vignettes. Since I got the airbrush setup, I am very much looking forward to seeing how much practice I will need to ‘get good,’ although I cannot imagine just dumping all my brushes, even if I do. Still, I am fairly certain that I won’t even be using spray cans even just for priming.
That ‘old hippie’ in me can only think about the harm to the environment they do. 
after that comment.