I was very involved with Garmin's development of GPS navigators for motorcycles from 2002 to 2008 (see attached story from Garmin's internal newsletter), but have not been active with Garmin since 2008. I do recall that each time I received a new pre-production device to test (these included the 25xx, 26xx, 27xx series, the 396 ad 496, and the very first Zumo) there would always be small issues between the new device and MapSource that had to be worked out and would result in a MapSource update before the product was released.
Back in the day, those of us who used GPS on motorcycles were very much the "early adopters" and we would create our routes on a computer and then download them to the device. Today, I think that a much smaller percentage of users create routes on a computer (ST riders being, perhaps, the exception), and as a result, Garmin does not get as much feedback from the field about little issues like this.
I agree with John Heath - the cause of the intermittent glitch when transferring routes from BaseCamp to a 660 or 665 is probably a result of Basecamp recognizing, working with, and downloading 'road attributes' in the cartography that did not exist when the 660 and 665 were released and/or the 66x series software was last updated. For example, today, most tunnels have a (invisible to the user) road attribute that tells the GPSR that the section of road is a tunnel, and to not expect satellite reception while in the tunnel. This is why today, with current hardware (e.g. a 590) you usually won't get a "Lost Satellite Reception" message in a tunnel as long as you are moving at a speed close to the posted limit for the tunnel. Instead, the device will show you continuing to progress through the tunnel, even though it obviously has no satellite reception. It is probable that older devices such as the 66x don't support similar recently-introduced road attributes, and when one gets used to create a route in BaseCamp and then that route is downloaded to the 66x, that's when the route fails to load on the GPSR.
Michael