"Length of service shows loyalty."

I agree, but it's not necessarily an indicator of the employee's loyalty to the company.  A job change on my (or your) resume might very well be an indicator of our employer's loyalty (or lack of) to us, or management's inability to run a fiscally sound business.  My point is that I wouldn't want to hold a job change against a candidate because of factors far out of his/her control.

"I have know some associates that gave very good plausible reasons for regular changes in employment but quietly admitted (but never to the boss) that they 'never stay in a job for more than 2-3 years'"

There are plenty of folks like that - and to them I say quit your day job and go into consulting   With consulting you get to switch up jobs regularly, which keeps the nomadic DBAs, programmers and cable splicers from getting bored.  Some might argue that consultants don't have as much "job security" as full-time employees, but personally I don't see a lot of companies touting job security as one of their prime benefits.