I see no issue with someone saying "Good luck everyone! I may be entering the contest, blablabla [INSERT STUFF RELEVANT TO THE THREAD]" every once in a while to actually show they have interest in the thread's topic. Simply saying "Good luck!" in 2903849082 contest threads is obviously ridiculous, as they are most likely attempting to gain a high post count, or sucking up for some particular reason. As well as this, it provides NO relevance to the actual thread.
I think someone saying "Good luck" repeatedly in threads should be considered spam, but adding actual relevance to the thread as well as saying good luck should not be considered spam.
If this were to happen, which I can assume would be prevalent for newer users, we have the duty as forum members to remind them via some sort of message that they should NOT be posting "Good luck!" "Have fun!" on every single contest thread in order to gain rep, a high post count or another stupid reasoning.