Gildarts, Laxus, Mystogan.
Essentially, you need to have your visible and invisible guild members. Gildarts is invisible, or covert. The idea is not to have him in the spotlight, so that he is not targeted, and remains like an Ace up one's sleeve.
Though this anime shows signs of the "never aging" effect, if it were geared more towards progressing over time, we can already tell by how the characters are scripted that Gildarts mode of protecting the guild - or his role in protecting the guild, is by not being in the forefront. That's what Laxus was for. Fairy Tail already has several S-Class wizards also.
It's likely Gildarts was never nominated for or applied for Wizard Saint. It's also likely that there is an exam/test involved just like the Grand Magic games. More importantly, while Gildarts feigns laziness, it's just a masque for him to be able to handle the more underground obligations of keeping Fairy Tail safe.
Notice how he was shown ???name??? that Laxus father was looking for, but Laxus was not. All part of the progression and the plan.