My Musing: Isn't it counterpoductive to Twitter a giveaway you are entering?

I wonder about this each time I see an option to Twitter a giveaway for an extra entry or two.  If even one other person sees your tweet and enters the giveaway haven't you negated the benefit of your additional entry?  And if two or more people see it, haven't you lowered your own chance of winning?

I can see twittering to promote the product that is being offered.  That's basically free advertising for the sponsor and it seems like a good way to earn more entries.

I can see twittering a giveaway to promote a friend's blog.  But I just don't see how it makes sense for the person entering a giveaway.

I love entering giveaways and I actually enjoy going to the sponsors site or shop to check out their items.  I like supporting the host and sponsor by following and liking them.

I'm interested in your opinion.  Is there something that I'm missing?  Does it make sense and I just don't see it?  Do you Twitter giveaways?