The guildleve limit on "quests" reminds me of Atlantica Online's system of "stamina" - after gaining a certain level, you could only initiate half the number of battles you could per day previously, however, the battle limit per day could be extended by partying up with more players and "sharing" their battles.
Guildleve is basically the daily life of an adventurer in Eorzea. First, the player goes to an adventurer's guild where they can take on a number of Guildleves. Then they gather at the Aetheryte with other adventurers or go it solo.
It makes sense that a casual gamer should be able to solo some of the guildleves. It would be ridiculous if you weren't rewarded for assisting/joining another character's quest, otherwise, why would you bother (aside from being nice)?
I just hope they have enough variety that the solo quests aren't simply limited to fedEx styled quests and provide modest rewards i.e. create soloable NMs with similar gear as a HNM quest, but with slightly lesser quality rewards (e.g. think of it as getting a NQ drop vs a HQ drop, perfectionists/elitists will aim for the white box whereas average player can be happy with NQ).