I came from a limited line dealer where we were able to focus, specifically, on two lines of lifts and know them inside and out. Especially with the advent of Tier II, III and IV software and the requirements thereof, it made things manageable. I am now with a multi-line independent dealer, servicing five different OEM's, and I have found that, both on the service side with having to try to manage training for technicians and having all of the necessary tooling for all the lines, and with trying to maintain parts stock and meet quotas, it is sometimes a losing proposition and we have to simply decide to take better care of one line and let the others wait/sit for a while. This is an enemy to customer service and has caused me some personal issues in trying to build bridges across the chasms we have.
While it is the way of the new world, I tend to lean toward focus and superiority whenever possible. And we all work on everything, and the comment above of having cooperative crossover relationships was spot on, when you can manage them (that is, when other dealers are receptive - not all are.)
Kudos to everyone above on their commentary - super topic and great information/thoughts all around.