To my mind, all photo contests are scams to some degree. Even most "respectable" contests (those in which there is some degree of "prestige" and the promise of a decent amount of traffic) are fundraisers that play to the undernourished psyches of would-be entrants. The question one should ask, I think, is "what will I get out of it". Meaningful publicity? Not likely. If there are prizes, cash or merchandise? Low probability when one considers the ratio of the number of prizes to the number of entries. Personal satisfaction? Maybe, only you can decide depending on your degree of undernourishment.
On-line contests are the worst, for the reasons cited by earlier posters. There is virtually zero chance that any contest participation will lead to anything else. The world of reputations and commerce are not structured that way.
The dominant question should be (which I have offered before in the forum) is "how much is it worth to me to play artist in the venue at hand?".