Yea, but that isn't really the argument. It's not about a specific player, it's about the integrity of the class niche.
I agree that if a player in your group is playing a rogue, than using your wizard spell slots to cover stealth and lockpicking is a waste. Where the issue arose is if the question becomes "We're going to do several different dungeon crawls in this game, I'm going to play a character to cover scouting, stealth, and lockpicking"; you're much better off (in terms of being efficacious at those roles) making a Spellthief 1/Wizard 4/Unseen Seer X as opposed to a Rogue X (or even Rogue 5/roguish prestige class).
In an ideal world, some version of fighter/rogue/cleric/wizard (or similar substitutes) would be the ideal balanced party. But in 3.5, a party of tank cleric/support cleric/2 utility wizards is simply much more flexible and potent that a party with a fighter and rogue in those spots instead.
This isn't some white room theorycrafting for me; I watched this happen in real time over the course of my years running and playing 3.5 and later PF1. By the time we got to our third campaign, a group that had seen a mix of caster classes and other classes had converted to all playing variations of casters.