While it could be fun for you to win or lose with a cup of soda, it is not fun to others when the winner goes around and boasts and laughs at you about his victory. Why don't we just play a match for fun without any betting?
To be clear, boasting and laughing at your opponents just shows you are not a true sport person (I'm not talking about teasing each other which is for fun and ... a little motivation ; ) Even in individual sports like table tennis if you want to become better you need good opponents - simple as that ... and there is always going to be someone stronger than you ... unless of course your name is Ma Long ... ; )
I guess 90 something % of my matches are like you said, just for fun. In case of a weaker player it is sometimes to motivate myself, here you have a goal, a free soda ... nothing tastes better than this ; ) So I also understand stronger players when they play me and want to bet a little, if it's 50/50 sure, if it's 70/30 for him I'll accept handicap points ... no problem.
In Poland 80/90 ties we had Andrzej Grubba and Leszek Kucharski as the best well known players. Leszek once said their rivalry played a significant role in their career. They usually played for a little money at the end of their training sessions ... kept them really motivated ...
But ... I also agree with you that the fun and enjoying the game is the most important here. Like I would never bet on the outcome of any matches during World Tours, EU Championships, any event actually. I like to enjoy the good game and analysis who has the best chance, post match analysis etc. ... betting would definitely spoiled it for me.