Regardless of WHy Sosa used the bat in a game, is was incredibly stupid that he did.
If you buy the whole crap about it being a "batting practice bat" then that only shows that Sosa was more interested in being the center of attention by hitting a lot more homers in BP than he was in actually getting ready for the game. If anything, a player should take a bat that makes it MORE difficult to hit - maybe a couple ounces heavier for instance- into BP to make hitting in the game seem more easy.
As for why he would only have one corked bat, the explanation is simple. If a corked bat should happen to break, they are going to probably look at all of your other bats. And if a bat should break in a game and found to be corked, you are going to be suspended for a few games, which is MORE thank enough time to simply cork another one. Therefore, there is no need to own more than one.
Personally I think everything Sosa ever said was nothing more than a calculated attempt to cultivate favor with the fans. He played the little Flintstone vitamins game, he magically forgot how to speak English in front of Congress, he did his little bit with McGwire where he kept referring to McGwire as the man, he played up to the dugout cameras, etc.
Plain and simply put - no I don't believe it. I believe instead it was simply a pathetic attempt at cheating by a player who loved to take shortcuts.