SMN is tricky. DRG, MNK, BRD and BLM with good gear can derp with their skills and still put up respectable damage. SMN, on the other hand, is really, really easy to mess up.

A well played SMN with matching gear can be equal to or higher than every other DPS class, depending on mechanics. The problem is that most SMN's I've seen have no idea what they are doing. Using Garuda to clear trash instead of Ifirt? Not properly timing Raging Strikes+DoTs+Contaigen on bosses? Having huge swaths of DoT downtime because you're not managing your timers? Casting the strongest and most useful DoT (Shadow Flare) last? Not managing Aetherflows properly? If a SMN messes up any of these, their DPS is going to fall off a cliff.

In any event, as it stands now, BRD is easily the most wanted DPS thanks to how easy they are to play (ranged, full mobility), but for all the other classes, especially SMN, it really comes down to play competence.