Here's a scenario that illustrates my concerns:
The party includes a 10th-level sorcerer and a 10th-level wizard. They defeat the monsters in a dungeon, and come across a stone door with an impression of scepter in it. Some Intelligence checks let them determine that they probably need to place a scepter of that shape into the door to open it. (Did they miss it in the dungeon? Was it not in the dungeon? Irrelevent!) So they set up night for the camp. That morning, the sorcerer switches out cone of cold for creation. He creates a scepter (stone, metal, whatever, they have coins and there are rocks around), which they use to open the door.
Deeper in the dungeon, they come across some sort of fiend bound in a summoning circle. It appears to speak a language, but not the same language that the party does. The wizard ritual casts comprehend languages. Now she can figure out what the fiend is saying, but can't communicate back well enough to for the party to be confident they won't be getting themselves into trouble (they don't read the same language, so she can't just write things down for the fiend to read). So they find another place to rest, and the sorcerer switches out haste for tongues. Now they go and have a conversation to figure out what's going on. The fiend claims to have a way to help them progress further in the dungeon (by dispelling a magical protection that seemed resistant to the party's dispel magic attempts). They party says, "we'll think about it" and retreats back a few rooms.
They settle in for a few days. The sorcerer restores his haste. Since they think this fiend is resistant or immune to cold, his player rifles through the PHB and Xanathar's Guide to Everything, and grabs synaptic static. He normally likes his 4th-level polymorph, but for this fight, he conjures himself up a banishment spell instead. That only took 3 days, but the rest of the party is getting restless, so they return. They release the fiend, and when things go poorly...banishment!
Continuing through the dungeon, see invisibility and enlarge/reduce also get temporarily switched in for the sorcerer, to good effect.
With the information they acquire at the end of the dungeon, they head out in pursuit of a white dragon. The sorcerer spends the 10 days travel time redesigning his spell list for the mission...protection from energy check! ...earthbind check! ...
I'm going to make the rather modest proposal that this is stepping on the toes of the wizard.
The biggest problem is that the sorcerer has the mid-adventure ability to swap in spells from his entire spell list whereas the wizard is limited to his personal spellbook.