My experiences would indicate that when a dealer carries one primary line the customer support parts, service etc ism easier on the organization in learning the ins & outs of how to "keep 'em" up and running. When a secondary line that is taken on that directly competes with the primary line the support behind the secondary product will not be up to the same level as the primary. Some reasons being - dealers have a limit on the amount of financial resources they are willing to support a new line - parts, new equipment inventory, rental fleet, technical training on the new line slowly ramps up, the whole organization has to learn another library of resources materials; how to & whom they communicate with; new working relationships, policies & procedures. It is always easier to fall back on what you know best, especially when companies are working with leaner staffs. Sorta' like finding a new girl friend - you have to learn how to kiss the new one the right way or she's on the next Greyhound out of town - except lift trucks will never be that much fun. That assumes you are not married.
And I have run into a dealer or two that will take on a second line just to take a "low priced" machine out of the competitors hands & by a few units, now & then and put them in their rental fleet.
I realize there are exceptions to what I have said & I can point to a few myself. But as it was once said It is hard to serve two masters.
Now for my however comment. However, if the secondary line is unique (like a Bendi or Moffet type or large capacity lift like Hoist) product that compliments the primary line rather than directly competes with the primary line take the above comments and send them to the recycle bin.