It has to do with weight aloft issues. In order to pivot and keep the raydome level, there needs to be a counterweight also held aloft...and because you do not want it to tangle with sails, that couterweight has to have a fairly short lever-arm, so it has to be considerably heavier then the raydome itself. Frankly, that is reason enough...BUT; The cabling that is actually designed for constant flexing as the raydome swings around is also much heavier and bulkier, so smaller vessels (which are also prone to care much more about weight aloft) usually do not run masts that have enough room on the inside of them to have all the halyards, the masttop wiring stuff and the raydome's springy cabling all fitting in.
Does having your raydome always pointing level and not alerting you to the darting of fishes and the flights of seagulls, YES, absolutely....But on a smaller boat it often creates more problems than it solves to have that optimal system...so you have to weigh it in the balance of your needs and wants. As a test, though, Find out how heavy the entire setup you propse is, add about 10-20% because they never include mounting bolts and plates and such. Measure off that much weight of water or something into a jerry can and using a downhaul, run that up the mast to the height you wanted the raydome. Secure the downhaul line and then tension the halyard as much as possible....Now go for a real sail....Can you notice any difference in performance and boat heel?
That may help you decide.