OK, so whats wrong with lubing like that? I have always done it that way. And that is the way most people I know who have been riding for years do.
But lubing it by spraying out of the can? Nothing wrong with that...
What's wrong with it? Well for starters, its fucking stupid that's what, and for a number of reasons.
1. It greatly increases the chances of you or someone else getting hurt (i.e someone bumps into you, you slip while moving around, something falls onto you/bike, bike gets knocked off the stand, etc).
2. You don't lube the chain effectively since you either miss the critical components of where the chain needs to be lubed, lube some areas not enough and others too much.
3. You fling off most of the lube before it even gets a chance to penetrate the critical areas of the chain.
4. You make a much bigger mess than it really needs to be.
5. Did I mention you greatly increase the risks of harming yourself or someone nearby?
6. The people you know who have been riding for years lubing their chain in the same way are even more stupid than you are for telling you its ok.
7. Why even take the chance, however remote it might even be? Doesn't make sense.