Low-lying rectal cancer/T4 definitely necessitates IMRT in my view (unless you are still treating anal cancer AP/PA!) since you have to cover inguinal/external iliac LNs. Anecdotally, I have noticed significantly improved GI toxicity with IMRT > 3D when you push the OAR limits on potential small bowel. I agree that it is a crapshoot getting it approved through payors although I learned that it is accepted as standard by Anthem (at least in my neck of the woods).
If I really need IMRT and the payor is still resistant, I sometimes send over IMRT/3DCRT comparison with the relevant dosimetric objectives highlighted. This still doesn't always work and it eats up precious time that you could be treating instead.