So I agree that multi-brand dealers are not ideal. If given the option, I prefer to purchase from a dealer who does just one brand. Who knows the bike better, who would have a better selection, who would have more factory support and who would be best trained to service the bike. I think without doubt it is the one brand dealer. Saying that, I think there are two reasons that multi-brand dealers are a necessary evil.
1. I like many of you research the hell out of a bike before making a purchase. We are here on the forums. We read the reviews (actually these days more watch them on you tube). We visit multiple dealers and test ride many bikes. When we are ready to purchase, we did our work and know what we want. It seems some of the new riders (not all) want to shop like they do in a department store. They want to go in see all the selections and types of bikes, test ride a few and make a purchase. No one has time for all the research these days and there is little brand loyalty.
2. In the case of a brand such as Zero. A one brand Zero dealership would not make it anywhere in the US other than possible California. One of the Zero dealers near me who has been selling Zeros for a few years now told me they sold a little less than 50 of them. This is not enough to keep the lights on in NY. They have to have more than Zero bikes to sell at least until electric bikes become the norm and Zero becomes a major player.