I played and started DMing in 2e, and I just do not get this mindset. Is it just flavor? Seems kind of restrictive. (It's why I avoid grim and gritty campaigns. Fun only for the DM.)
Not really. Having run g&g with that type of restriction? Players like it. When you have to work for spells. Duelling the firecaller for access to new pyromancy, working with an apprentice and realizing 'wait, if I channel just so I can do...' and learning something new is fun. There's also a lot of flavor built up when each wizard doesn't just spam the same 5-6 refluffed effects. Also...
Is it about game balance? Because I think that's a terrible way of doing it. If a spell is overpowered or troublesome, rewrite or ban it. Never giving a wizard a scroll of Rope Trick seems a poor way of "balancing" the spell.
If you provide a single use for a spell or a 'once in a lifetime' alteration of a spell due to a wild magic area, specific star alignment, whathaveyou? It makes it interesting. A magic rope that is imbued by a fakir in an Arabian market, or a box that allows access to a dead mage's sanctum for a few hours can serve as a Rope Trick 'scroll'. Just a matter of flair.
Slainte,
-Loonook.