BLM isn't part of any infinite MP conversation because their MP is quite literally a nitro tank. SMN is better compared to RDM and the healers as it is basically rezzes that blows through their MP. In the case of RDM and the healers, sustained AoE and GCD heals will also do this.

I will say that SMN has a ton of functionality outside of its MP resource. Probably the highest out of all the casters, but many abilities such as the demi summons basic attacks are tied to GCDs that use MP. This doesn't necessarily put a strain on their MP, but it does create a demand for it. This makes them every bit as screwed if they get KOd or do go OoM and Lucid or other means of MP restore are not available.

It's important for all casters to manage their MP in a way that sustains the finite resource that is MP. SMNs, and on the healing side WHM have a much easier time doing this because of abilities within their kits.