As a PT main I have a lot to say on this topic. Here goes.
"Divide your time"
This is by far the most common argument I hear against playing multiple characters and it's one that has never really sat that well with me. Sure, if all characters were technical like Joker or Peach I'd possibly agree, but different characters require different amounts of time to master their character-specific skills. Fundamental based characters such as Ike or Bowser with simple combos may take more effort to master then many people give them credit for but they're still basic enough to study as a secondary that I can't imagine picking one up hurting
too much.
He also mentioned how it means there's more match ups to learn. Not only is some match up knowledge transferable between characters (similar to fundamentals), but if you're picking up a secondary for specific match ups then you only really have to grind out and study those specific match ups. While it is more information to learn I don't think it's as big of a problem as people treat it like.
"Cheese the player"
I'm glad he mentioned this as I often see "match up inexperience" listed as a reason to pick up a secondary. I agree with him here - it might work in the short term but taking this approach will only hurt in the long term. It's not sustainable. I think if you pick up a secondary, you probably want to know what match ups those secondaries are for.
"MUs don't matter THAT much"
I agree with him here as well, at least when it comes to high / top tiers. As a former
main however, I can assure you that some characters further down the pecking order will sometimes have more divisive weaknesses and might require a secondary. He's speaking in the context of a players who want to get as far as they can, they probably shouldn't be maining characters like that in the first place, so I'll give it to him on this one.
As for my personal stance on the issue?
I think secondaries are doable and can come in handy at more middling levels of play, I don't think that people should feel pressured to pick one up. While I don't fully agree with it as a reason to avoid secondaries altogether, the "divided time" argument still hold some truth to it and playing any secondary to a degree of competence is going to be a large commitment. In a lot of situations it might just be easier to learn a single bad match up with your main then it is to pick up an entire new character just so you can counterpick it.
On the other hand though, since I've started maining Pokemon Trainer I've started to believe that studying other characters can help you improve your basic fundamentals and general understanding of the game. Playing Squirtle has helped me gain a new perspective on the importance of aerial attack angles, for instance. I think there might be some value in learning secondaries and pockets for that reason, especially if they have a different play style then you're used to (but then again, having a different play style then you're used to might make them harder to learn).