My preference is for the Wizards’ Guild to be a secret society (or sets of secret societies), as opposed to an easily accessible professional organization like a Plumbers Union. This makes it difficult for PCs to just wander into a town and get items analyzed, Cleansing/Resurrection spells cast, and rent access to a library so as to learn Summon 7-hex Dragon or whatever. Instead, I prefer to have wizards need access to rare texts to learn spells, requiring them to either go find the books and keep them safe from prying eyes and filching fingers, or develop a relationship with someone who has some occult tomes
as outlined over on my sporadic blog. This give me much more control as GM over what spells the PCs can actually learn, as opposed to having a player tell me, “I have 500 XP, so I’m learning Astral Projection.”