I agree, why do you need to reduce the frequency of backups? If you don't have it plugged in all the time, you don't have to worry about missing backups as Time Machine caches what to backup and does so the next time you plug in the external drive.
If you're worried about disk space, again, you have nothing to worry about. Over time, those hourly backups are consolidated into daily backups, which then become weekly backups, and I suppose after long enough they'll become monthly backups. So your drive isn't going to become cluttered with hourly backups over time.