First off: Loyalty is not depth. Good depth in a single problem of courses requires that you stick to one place and be loyal. But great number of times people do same repetitive work without learning anything new or enhancing new skills. Such things is NOT really better depth.
Many a times, I do find candidates who have been around for more than 5 or 7 years. This is indeed great. But then you look at the depth of his/her work. Has the person continued more projects, higher up in the responsibilities or has been stuck without any significant growth? Or has she/he been only stuck at a position.
On the other hand there are candidate has been almost always changing the job, say every 6 months or 1 year; If people jump too often it is a sign that they don't stick to some or most problems too long and or have severe problems with aligning the work place culture and hence cann't stick around. Such people might run away in the middle of the project keeping jeopardy. People who have left critical assignments just because of more money or bad days of company, it is a sign that you cann't quite trust that person to fight in a critical project.
Of course, of many changes, you might have took up one or two odd positions where you might find yourself misfit - but that's not that bad.
So - a right measure to judge whether jump that people took should be reasonable.
In one of the books by Jack-Welch - Winning, he writes:
..why the candidate left his previous job, and the one before that."
This "tells you more about them than almost any other piece of data."
(p. 96 - Winning)
The essential answer he gives is that when people change because of desire the learn more and do more exciting challenging stuff - it is the right candidate.
If the candidate has continued growth and a long tenures (fewer jumps) it is really ideal hire. But all else depend on his individual performance on certain projects.
If the person has at least one or two great projects, it is a reason to believe that he will be willing to at least attack the work and finish things and will fight for challenging problems.