My biggest reason for standing is because there is not always a stage. I feel that the performer needs to be "spotlighted" in some way, and if there is no stage, the least I can do is "rise" up to the level of their eyes or above.
Another major reason is the singing issue. There is no doubt that my lungs work better when my big gut is extended, instead of crushed up in my lap like a basketball....not to mention the overall look is better. Standing allows me to feel the pulse more, move to the beat, see further out into the crowd. In a background situation .... anything is OK, but for a "show" I recommend that everyone be as visual as possible. Remember, Billy Joel and those guys are not performing solo ... there are other players, lights, effects, etc ....

If it's just me and the crowd - I need to be in the best position to control the flow. Sitting just makes the statement that it's less of a show and more background.

BTW - I DO sit at certain places ...... like I said, It's an issue of show vs. wallpaper. Sometimes, I'm the paper too.