Not all players have PC hardware capable of running the FFXIV client, and downloading it takes time as well. However, your suggestion of a "PC Guest Pass" sounds like a good idea. It would not be necessary to separately apply a pass to each account if Square Enix has a way to universally disable the PC license entitlement check.

For practical purposes however, PC licenses are already practically free or even better. They are purchasable in the NA region for as little as $10 and come with 30 days game time which is worth $10 to $15 itself (Legacy pays $10 a month, 30 days Standard sub is $15). Many players have indeed taken advantage of this during the recent PSN℠ outage.