It would definitely be more realistic with this implemented. also if you change weapons your primary rifle/MG/SMG shouldn't be slung on your back. not sure about other nations armed forces but in the US military most of us ground pounders use one or three point slings which make it so when you need to switch to your sidearm quickly you just let the primary go and the sling makes it rest across your chest or at your side. and yes we are definitely trained to change weapons on the move, in the marine corps we call this a crossover drill. in regards to launchers though if its allowed it should appear to be incredibly awkward.