As the cassette cog wears or your chain tensioner bounces, you might experience chain derailment or slipping as the cassette cog is usually cut at angle to facilitate gear changes. For some this may not be a big deal. In some instances this could cause an injury if you slip, fall or otherwise cause the bike to mishandle as you apply a large amount of force like climbing a hill for example.

A BMX or individual rear cog is preferred for safety and longevity.

If you have the right range of chain tension, the difference between wear rates on larger versus smaller chainring + cog combinations is not onerous. Just buy a new chain when you need to.