Sim trading is not the real thing for sure but is the next best thing when you are perfecting you're strategy. It all comes down to how you use it and whether you use it to trade in real terms or not.
Its like a box of matches or a knife - you can use it or abuse it.
I am from a Particle Physics background and I know the value of a simulation. A simulation is ony as good the person who wrote/uses it.
FWIW Demo are not the same as Simulators. Demo accounts are something completely different and allow, like you said, to try out the interface only. Simulators can be software based or website based and are for learning and testing strategies and vary in sophistication. For example, I use Ninjatrader 8 software which has a simulator built in which allows for slippage, fees during fills etc