Many years ago I used to do ATC. I will admit to being a bit 'relaxed' on the radio on occasions when I knew it would not cause any problems for anyone and was equally happy if those on the other end of comms were similarly relaxed at times. Life without a little bit of variation or fun can be decidedly dull.
However, when it got busy, there were non-native speakers on frequency or someone had a problem, I went 100% standard phraseology as did the vast majority of others on frequency. For me, it's a matter of professionalism - knowing when it has to be done right....and doing it right. Some may differ and believe that it should be done right all the time, and I really couldn't argue with that position. But not recognising when it need to be done right, or simply never doing it right, is unprofessional.
BraceBrace has it correct when it comes to monkey see, monkey do, and if someone never learns what is correct in the first place, they will never be able to demonstrate professionalism when it is absolutely necessary.