Putting in the effort to show up, supposedly give it a shot and then cancel your score for whatever personal reasons (that you don't have to explain if you don't want to, adcomms really don't care much about ONE cancellation but want explanations about 2+) looks better than flaking out and having a no-show 'absent' on your score report no matter how you look at it.
Adcomms tend to be curious/suspicious about and want a good explanation/excuse when they see an "Ahead of time I registered and paid good $$$ to take the test that influences my future but didn't show up at the location with a few pencils that morning" 'absent' note on a score report.
Giving it a shot, having a bad day and deciding to cancel is understandable, everyone has bad days. However, unless you are incapacitated (like being in the hospital) or are otherwise physically incapable of making it to the test center for a verifiable reason (sudden military deployment?), pulling an absent has FLAKE, can't handle the stress, pressure and demands of LS written all over it.