The skills your practice are the skills you improve. The things you never practice are what will atrophy.
If you mean bad for your chess as in bad for your tournament play, then simply compare. What goes on in an OTB tournament game, and what skills do you use primarily and what goes on in a blitz game and what skills are needed?
i don't think blitz can really harm you chess developement as long as you don't play your tournament games like you play your blitz games (lazy calculation, dubious sacrifices, playing fast for a time advantage etc.) because that would be harmful
now bullet on the other hand probably is really harmful because you aren't really playing chess, you are just trying to chuck wood around the board faster than your opponent and win on time
As a coach I only recommend Blitz to students when they need work on openings because it forces you to play moves & variations that you know & get more experienced with them.
It doesn't so much harm your chess as it stunt's your growth & improvement. you will find that you cannot play long games the same and you need to retrain yourself to think in a more efficient way after playing a lot of Blitz.
My advice would be to try playing friend's or tournaments with G/60+ time controls and try spending a couple minutes on each move. You will play better & improve.
If you just want to have fun with chess and don't care about improving, than play all the Blitz you want, but as for improvement, you need to regulate your Blitz play and focus on long games.
~ Luke
I just played a ten minute game, and found we were both riddled with mistakes and made stupid moves due to time contraints.
Do you think blitz can harm your chess development and do you think its better to stay away from it or do you think it can be beneficial to your chess development?
Best wishes
Dawud Beale