Chimchar got a fair amount of screen time under Paul.
Ash didn't own a Piplup, but that line was thoroughly showcased.
May's Torchic got a decent amount of screen time for a non-walking Pokemon.
Overall I think the only lines that were absolutely lacking in appearances were Totodile's. It seems like we've seen enough of the others not to miss them.
You seem to be under the impression that a Starter absolutely must go to Ash to have any significant amount of screentime. As history has shown us that's simply not the case. That works both ways, too. Even if Ash catches a particular Starter it's no promise that they'll amount to anything, let alone get screen time.
Recurring Character- A character who shows up every once in awhile in the anime.
Main Character- A character who shows up in nearly every episode of particular series.
Iris and Cilan do not qualify as recurring characters.
And you did not so much as imply that when you said one would go to Ash's travelling companion and the other to Ash's travelling companion, both being main characters of the series.
I disagree with travelling companions for their own reason.
But again for the purposes of THIS argument, we're talking about the final starter being relegated into a recurring character. Someone like Paul, Barry, Conway, Trip, Bianca, etc, etc.
That starter no matter what you believe will be screwed over in some fashion more so than if it was on the main cast (Ash or his travelling companions).
The writers would never do that, just like the writers would never decide to not give Ash a starter for pretty much the same reason.
Ash is supposed to represent the player of the games the most. It doesn't make sense for Ash not to have a starter BECAUSE you are forced to choose a starter in the games, what YOU do after it is your business, but Ash is going to get one starter.
The reason why Ash sometimes gets a lot of starters is because while Ash represents the player character, it would be wrong to set one starter over the other. Now that obviously happens later on, but after everything settles in.
For example, in Hoenn, when I chose Ruby, I chose Blaziken, probably because of Harrison's, and it was the most focused starter, and it looked really cool (even if I had the most stupid reason to dislike 3rd gen when I found out about it).
Then comes the anime (when I was able to watch it again), and what did Ash have, he had Treecko.
I never really focused on the Treecko line, until I saw Ash's Treecko/Grovyle, it clearly was better than the Blaziken line. There were other reasons why I liked Sceptile over Blaziken, Better design, and just much more badass.
So it was initial hype that caused me to choose Blaziken, but it was focusing on Sceptile through Ash's Treecko/Grovyle, and other reasons, that I really began to like it. May's Torchic/Combusken was treated well, but not in the same way as Ash.
For someone who likes Pokémon for it's battles, primarily, it's clear why I prefer Sceptile over Blaziken. Blaziken may be a fighter, but Ash's "Grovyle/Sceptile" is BEAST in comparison.
That's primarily why I disagree with anyone other than Ash having all starters. Because one of the starters just gets screwed over, EVEN if Ash has them, they still get screwed over.
But at least Ash actually uses his Pokémon moreso than any of the other characters. Took May forever before she used Torchic (like evolution), and while Piplup was "overused" it's not changing my opinion on Prinplup/Empoleon.
And no matter how screwed over Torterra was, I still think it's cool that Ash got a freaking giant land mass Tortoise, which is his current biggest starter Pokémon, and probably equal in size to his Snorlax (if not slightly smaller than Snorlax).
I just think I have higher opinions of starters if they go to Ash. Which is why I dislike other people having them, and only them.