The problem is that in more advanced systems, by selecting Standby you disable TCAS. If you simply twirl the numbers, the system puts itself into Standby without disabling TCAS - it still operates.
If, for some reason, you get distracted by another (more urgent) job, there is a danger that you may forget to put the Txpdr back to ALT/RA and not only are you then bereft of TCAS advisories, but so is anyone else into whose path you may blunder.
On the more simple systems - the type of King Silver Crown gear you find in little piston-engine puddlejumpers for example - the level of sophistication requires more care to operate. But applying that level of care to systems with more sophistication holds additional dangers.