Its not bad to change the fluid frequently as long as its done within the recommended time frame.

If you have an older vehicle that has never had the fluid changed then what happens is the additives break down as well as the trans clutch friction materials wear off due to the lack of functional additives. The friction material becomes suspended in the fluid and almost acts as a friction promoter for the worn clutches. So when you change the fluid in this case, you remove all the clutch material and the new fluid cleans off the worn clutches.... and usually this combination results in a slipping trans.