I think the main reason for diagnostic imaging for long-since post-op spines is the determine the cause of pain/problems. If you don't have any symptoms, you would never undergo any invasive treatment just based on some abnormal radiologic finding. So it would seem like common sense that you don't need to have any more radiation right now if you are not having problems.
I had a fusion of L4-S1 in 1984 for spondylolisthesis. I had no imaging at all of my back for the past 24 years because I really didn't have any problems. I developed pain and tingling about a year ago, and was shocked to find out I had a pseudoarthrosis at L4-L5...I assume it never healed properly. But I am glad I didn't know about it, because I would have never considered any type of revision surgery in the absence of major symptoms. I am still not ready for surgery!
I imagine if your original surgeon thought you needed regular follow-up over the years you would have been advised at that time.
Gayle, age 47
Boston brace as a teen for AIS
Oct 2010 fusion T8-sacrum w/ pelvic fixation, TLIF at L4/5.
Feb 2012 major A/P revision for broken rods
mom of Leah, 13 y/o, diagnosed Jan '08 with 26* thoracic JIS (age 6)
4/08 26 degrees, brace 16 hrs/day
9/08 17 deg. OOB - brace discontinued
9/09 30 deg, resuming brace
5/10 VBS Dr Luhmann Shriners St Louis
4/15 stable 5 yrs post-op, 18* T/ 22* L, Risser 2
also mom of Torrey, 10 y/o son, 16* T, stable