There is also the issue of time. We all have busy lives, and it can be a big commitment to enroll in another style of dance or another activity. Lets assume that the goal is to improve our bellydancing by practicing ballet arms. You'd spend hours in ballet class, no doubt getting many benefits, but a comparatively small amount of that time would specifically be spent on arms. Then you'll need to integrate what you learn there into your bellydance practice, as well as practicing the other things you will need to be able to keep up with the class. If you goal is to try out a ballet class and maybe improve your arms while you're at it, then this is a GREAT idea. If you are JUST looking to improve your bellydancing.... it might not be the most efficient use of your time.
Ultimately, that is what this post is about: don't go off label and prescribe a course of action for its side effects. I think we do this because we somehow think it will make it easier, we are looking for a hack, a short cut, but you know what they say about shortcuts.
Maybe it's the New Yorkers in me, but I prefer the direct route. So if you're trying to improve your undulations you don't need to enroll in a swimming class and stick around until they teach the butterfly stroke, just practice your undulations for a few minutes each and every day. If you want to learn the maqsoom rhythm: skip the African drumming circle (unless you wanted to go anyway), practice the maqsoom on your finger cymbals and with dance combinations that capture it. Working on your arms? Go ahead and take a page from ballet and do a port de bras, but use your bellydance arm frames and integrate it into your isolation drills instead of worrying about which number position you're in. There is nothing wrong with cross training, or with borrowing a good practice idea but: don't become so focused on the means that your it takes away from your ends, don't make things more complicated than they need to be, and don't forget that all you need to do to become a great bellydancer is practice bellydancing. Lastly, remember: