All this bad main stream thinking that Summoners have superior mobility...

Let's line up the summoner spells:
Bio - Instant
Bio II - channeled
Miasma - channeled
Miasma II - instant, short ranged
Shadow flare - channeled
Ruin - channeled
Ruin II - instant, 59% higher MP cost

What would give summoners "superior mobility" would supposedly be the instant spells. There are only three spells that are instant: Bio, Miasma II and Ruin II. Bio is a DoT, which shouldn't be spammed at all and should not be clipped prematurely. Miasma II has to be in melee range. Most of the time walking for Miasma II is already lost walking back to your original position. Add in the fact it's actually less potent than Ruin (II) and should not be used unless in tandem with Contagion.
Which leaves only Ruin II that's reliable to use for "mobility".

This "mobility" is windowed, however. A summoner has to reapply DoTs frequently: Bio II every 30s, Miasma every 24s and Shadow Flare every 30s. The timing of application and re-application aren't static: You don't cast Bio II -> miasma -> Bio in this order every set interval in the fight. This makes the window for mobility - Window when summoners have no need to channel their DoTs - very irregular.

Movement forces the summoner two things:
Post-pone their DoT application: Which is a DPS loss
Cast Ruin II more frequently: Which will cause MP issues. A case that's more obvious in FCOB.

The exact same movement restrictions black mage have also applies to summoner. Except that Summoners don't have Aetherial Manipulation to cross great distances within one GCD window.