For me, the main reason to do the retirement extension (versus the marriage ext) is the ability to be totally self-reliant.
Processing the retirement extension requires only you, the ex-pat, and your Thai bank account. No one else needs to be involved, and that reduces potential complications or mishaps.
In general, I think successful, retired ex-pats -- wherever they settle, should be able to do all the necessary administrative procedures required by the adoptive country.
If, having mastered those procedures, the ex-pat may then wish to use the services of an agent or other helper to save time in preparing the documentation, standing in line, or other nuisance.
But, in sum, Rule #1 of a successful overseas retirement is this: Know the local rules, and be able to fully comply with those rules on your own, with no help or involvement of anyone else.