Beta testing is helping the developer to make his product good. When more people try your game it is possible to find strange things you won't find if you only try the game at your workspace (for instance, related to sound card properties, internet connection problems, old operating systems).
You will not find Trojans in beta's, if the developer is serious about his product.
Funny story about Richard and I in our very beginning of audio game programming. In beta testing stage of Sneller, we found that the score-system of Sneller was only capable of reading scores up until 35000 points. Although we ourselves never reached this score, the gamers that played Sneller managed to get 59500 points! This resulted in a score of five...hundred....points. The gamers did not understand why they only scored 500 points, because the score system never said fifty-nine, because it was not present.
Then you've learned something: do not underestimate the powers of the real audio-gamer. Always reckon with the unexpected! We never thought the score could reach such a high number. So, we had to record new score samples! Fortunately we already had to, because we were translating the game to "Drive" 
By the way, yesterday I made a small score-patch for Drive, so that the score system works with a real hall of fame at the audiogames Drive page. Although I know nobody is playing Drive at this moment, everyone is far too busy playing Top Speed 2!
Have fun!
http://www.audiogames.net/drive/
