From the little I know about BJJ, it seems to teach flowing from one movement to another when resistance is met or a technique is thwarted; its about structure, and leverage versus pure force against force. If I was ever going to cross train in another art, it likely would be BJJ for that reason. However, for me, I doubt this would happen because learning ONE art as an adult with a career and family is an uphill battle enough as it is. Theres only so much time in the day and I would think my WT would serve me better if attacked by an angry drunk guy in a bar or some punk on the street. I suppose I could always just ask my BJJ friend who is a blue belt to show me some basics.
I hear conflicting opinions from different WC / WT guys about stuff like cross training. There seems to be the "Wing Chun is a complete system and has everything you would ever need" attitude (my paraphrasing). Some admit there is value in cross training in other systems as long as it doesn't fly in the face of WC/WT principles (cautious but logical), others like Jin Young (ChinaBoxer on YouTube) think WC and BJJ actually complement one another as seperate but harmonious arts (intriguing perspective in my opinion).
I've only had a little direct exposure to Wing Chun, but from what I've seen the underlying principles don't clash with BJJ at all. (As you say: leverage, structure, and flowing around resistance are key to both arts.) The basic tactical doctrine (preferring going to the ground vs staying up) is different, but that can be easily resolved by using Wing Chun while standing and BJJ if the fight goes to the ground.