I wouldn't mind betting that for 'most' of us we start off being strict and careful about what eat and that over a period of years, maybe decades, that initial enthusiasm becomes harder to maintain. In that respect I can see that IIFYM is a useful, much more likely to adhere to, way of eating. I suppose the best way to do it is make sure that what fits into your macros is still reasonably healthy.

Personally I hate or at least don't really ever want to eat tuna again. So I substitute something that isn't as low in fat and therefore will probably never have a six pack again. Its not just tuna. That's just an example of having eaten 'bodybuilders' food for far too long.

So for me, if it fits and its not too unhealthy, I'll eat it.