So far, for me, (I might be wrong) there is no reasonable argument against this statement "When an ordinary listener discovers a song that he likes and has possibility to listen to it several times, he loses incite to purchase this song." Except that these issue does requires attention "I tried listening to this guy's song yesterday, with the watermark. I couldn't listen to more than 15 seconds of the song before I turned it off. It's annoying as hell (not the song, the watermark). Great marketing." But this is more the matter of polling issue. But in any case audiojungle have built their business and watermark for them is a must. I do not think that they are stupid as their business is large.
Anyway - my advice is - do not give away free samples to get popular. The one who gets free sample from you will never buy your mp3. It is up to you.
The overwhelmingly biggest flaw in your dated thought process and silly approach is that you are "marketing" your unlistenable snippets of crap to fellow nobody musicians. We're the most jaded and judgmental bunch there is. You need to be pushing your shit on the unwashed masses. The musically dumb. The listeners, not the creators. They're the people that could potentially buy your hopes and dreams.