Nah, the people who are against them in walkthroughs are against it because well... your second sentence just isn't true

We've had 3 tamarin mods and 2 marmoset mods for ages now and they by no means look any different in terms of climbing than any other animal in the game. They're not more buggy, they don't climb weird compared to the other smaller primates such as the lemurs or capuchins; they act exactly the same, they're just smaller.
I feel like the main reason people use for tamarins and marmosets in walkthroughs is the "they're small so the climbing won't work" but that's never been based on anything and can easily be disproven by just downloading one of the mods.
Honestly, Frontiers approach to walkthrough exhibits seems to be "animals we can't get to work in habitats because of complex ways of moving", which was basically confirmed in their dev journal. So I sincerely hope that's what they're going to keep using them for; because I don't see a reason at all to put small tortoises and all the aforementioned small mammals in a walkthrough when it limits your creativity as much as walkthroughs do.
That said, I mostly want some more bat species such as flying foxes. I think I would have preferred a flying fox over a sloth in the last Tropical Pack even