I only know the percentage MP gauge, besides my short time playing like 3 years ago, but I think it's effective at keeping MP as a resource that needs to be managed as a caster. I have not unlocked RDM yet, but as BLM I don't find too many issues with running out of MP. If I do, it's my fault for not keeping up my rotation properly. I think maintaining your resources is paramount to competent gameplay, otherwise HP and MP pools are impotent.