I've entered plenty of "(re)tweet this to win" type contests. The barrier to entry is incredibly low, and I've won enough times to keep myself encouraged.
If your time is so valuable that taking the time to (re)tweet something will be a drag, or if your twitter account has some intrinsic worth that could be brought down by this type of tweet, then yes, there's no reason to enter, but then again, if you meet either of these criteria, you probably don't even need to enter contests...
Well... a tweet saying "I saved space with some app" is probably not any less valuable to your followers than one saying "Mmmmm... fried cheese... [insert instagram pic here]". I see a lot more useless food tweets than I do contest tweets.
Edit: full disclosure... I frequently tweet food pics and almost never tweet content entries. :)
If your time is so valuable that taking the time to (re)tweet something will be a drag, or if your twitter account has some intrinsic worth that could be brought down by this type of tweet, then yes, there's no reason to enter, but then again, if you meet either of these criteria, you probably don't even need to enter contests...