With the exception of chain maintenance (addressed above), I'm assuming that the preference for chain drive vs. shaft drive on your LD rally bikes, WC, is its simplicity; one less thing to worry about underway?
Well, truthfully, no, that's not really it. I don't really have any problems with shaft-bikes in LD events per day, though I would think differently about that if I suffered from Beemer Kool-Aid Syndrome.
No, the fact of the matter is: with few exceptions, most ultra-performance machines are chain-drive for both the weight savings and the slightly more efficient energy transfer. So if you want the performance of a Blackbird or Hayabusa while rampaging on Endurance rallies, then you gotta be able to deal with the ass-pain that comes with the chain drive (which, for the most part, amounts to just the ongoing oil fling-off. This is simply the nature of the beast, and a necessary evil.)
That said, the Pro-Oiler offers the least fling-off of any automatic oiler on the market, because you can get it "dialed-in" to achieve just the right amount of flow to get the job done, and no more.