People have asked me many times how and why a musician can perform in more than one band at the same time. Of course, we are talking about professional musicians here, and only those who aren't freelancers. I myself consider to be a professional musician who also freelances if an profitable opportunity comes my way. But otherwise, purely for the purposes of artistic creativity, pleasure, and satisfaction, you should have just one band - or so the world thinks.
Such comparisons are probabably drawn from other fields like regular employment, professional sport etc. I consider music, especially that in a band, as a radically different field primarily because of the possibilities of collaboration, collation, and calibration of different ideas of a variety of forms, such as musical, lyrical, presentational, and performance related and many more.
Different individuals and personalities with varying ideas operate at multiple levels during the process of writing, arranging, and performing a song. The process itself creates environments that could be so different between apparently 'similar' bands (to an observer, that is) so as to provide a refreshing experience for the musicians involved. Ah, the wonders of creative freedom and liberty.
This is why I enjoy what I do - play with two different bands, jamming with a set of musicians to possibly form another band, sing in a choir, write my own songs and perform etc etc. This liberation keeps my spirit alive and presents opportunities for the occurence of moments to cherish. I adore this and I would never want to not be deprived of my freedom for creative variety!