You literally just defeated your own argument.
If you're "Very likely to cover one or both abilities in a party of 4 already" then there's not much replayability is there if you simply already have access to those skills. It's not like these situational, out of combat, skills particularly define gameplay of a class.
What is this, debate class?
No, the argument is that these abilities are already quite common. There's no need to scatter items everywhere to make 110% sure everyone everywhere can use them constantly. They're also not necessary to beat the game so why trivialize them in the first place.
Another argument is that the abilities themselves are devalued if you are expected to be able to use them every single time a target is available, even if you don't have the appropriate classes.
Perhaps most importantly, there's multiplayer where it would be really cool that your PC could contribute something unique to the party. But these class abilities are not flavorful or unique if anyone can just use a scroll or potion to gain them.
The only counter-argument that I can really think of is "they're fun to use, I like that I can use them whenever". Which is valid and fitting for some casual fun gameplay, but completely misses the points I made.
