Swiftness has a clearly noticeable effect on movement speed, swapping it for Mobility causes a noticeable downgrade.
Mobility description says it reduces turn cost of movement by 30% if no charge abilities are on cooldown. This clearly implies that having no charge abilities that can be on cooldown means that Mobility should be always working.
But it doesn't seem to work like that - neither on my charge-less summoner, nor on the electric bird demons that commonly spawn with it (those also have no charge abilities).