I recently added a generator hub and a set of lights to my touring bike. I love having them, but I commute daily and like the convenience of not recharging batteries all the time. I bought a Luxos U so that I would have a usb port available for charging all the time. I haven't toured with it yet, but I will this summer. I can tell you the wheel is heavier than what it replaced, and there is some residual drag, but neither of those is really noticeable. The new Shutter Precision 9 series hubs are small and light, with very low resistance, so building a wheel with that hub may mitigate those issues. That is my plan for the summer.
Personally, I like having the usb port and being able to charge electronics. YMMV. I can't see a reason to spend the money or add the weight with the full set up if you are not going to be using the lights full time like I am. I have toured with a battery pack to keep things charged and that was fairly easy; even took it on a recent backpacking trip (still questioning that idea). Depending on the electronics array on the next tour, I may take the battery pack to charge everything and let the hub charge the pack. Then I will have added even more weight to my set up. If so, did I really help or hinder myself?
If you go for it, I would build one very good wheel to use all the time rather than building two so they match wheel sets.
Hope this helps helps.