Well they make their job easier and just use it where they can't get shape perfect or too much work is required ,
For one they take complete payment and complete work should be done in exchange, not a hack job me and my m8 would do over the weekend.
Secondly the car only looks repaired but is actually the same way I.e compromised so in case of a hit car will get totalled and it will lose shape over time as the material will not have anything to hold on to.
And it is very dense so it makes the car heavy, so in lighter and less damaged areas it's fine but to try and fight rust or repair major dents instead of just minor deshaping is not the right way
Conclusion: everything has a specific use and it's fine as long as you use it for that purpose otherwise you are trying to put yourself in trouble, and you are settling for a temporary solution instead of a proper one, I might not have adequate experience for other things , but dents ? Yeah I've been there