I don't think you can apply that logic to the tests. Just because I have a few guesses on a test doesn't mean I'd do the same in an entirely different context, i.e. on the job in ATC. Personally I think it's the logical thing to do, to try and make some educated guesses when running out of time / unsure on a test, because it's a test and you want to prove your aptitude on that by getting as big a score as possible. If NATS are penalising guesses that would be foolish, you'd want people who make the best of a situation which in this case would surely be getting the most marks as possible even if it means guesses.
When I applied last year I answered about 25/40 cube questions but was sure I got them right. I did get through that stage. This year I guessed some so I managed to answer 39/40 questions, if I don't get through this time I think they will definitely be penalising the guesses (as well as creating one very angry wannabe controller!
).