The original toy existed before season 3 of the cartoon did, meaning it was created independently of the show's in-universe origin story. Hasbro made the toy on their own, then the show had to come up with its own explanation for it afterward.
And when that toy was released I was eleven, the internet was something my parents used, and I was first introduced to the form via Beasties when Optimus Primal got it after taking in the Spark of Optimus Prime in a very Matrix-y looking container.
I don't think I saw the toy for another couple of months. Canada wasn't exactly known for the best distribution. I think Canadian Walmarts are still waiting on their RiD Scourges
Point is that, to me (and I've only ever defended the PotP toy from my perspective), this form of Primal was specifically a very special upgrade he got because he took in Prime's spark. That episode is lodged in my brain as one of the more important episodes of TF tv, and as such the importance of why he got the upgrade always seemed more important to me than his quad-changer status.
'cause I never had that original toy. It was big, expensive, and a character my parents had bought three times already (four if you consider they thought Optimus Prime and Primal were the same guy).
My longing for a version of that form wasn't based on the fact that he could awkwardly form a car made out of robot monkey parts. It was based on being the body that Primal got when he held Optimus Prime's spark.
Later on in uni I gained some more appreciation for it when I binged Beast Machines. Which was primarily concerned with the jet mode.
So when PotP Optimal Optimus came along...
1 he fit the theme
2 he allowed me to get a version of this design without breaking the bank on an aftermarket figure in good condition
3 he did everything I needed him to do (robot, gorilla, jet) plus a little extra (a fun Cybertronian mode Primal)
I'm not saying you're wrong for feeling like it didn't meet your expectations. I'm just saying I'm not wrong either.
What we get out of these characters and designs is ultimately personal and context dependent. It's the Ultra Magnus issue. Do you value him as a functional car carrier or do you want the figure to focus on the armour up gimmick? There's no right answer. Just whatever about the design/character resonates with you.
For me what resonated with the Optimal Optimus design was satisfactorily represented by the PotP toy.
The funny part is... I didn't even vote for Primal. I wanted Star Sabre, and had to pay crowd funding prices to eventually get him.
Primal wasn't even on my backup list (I wouldn't have minded a Hound W). Yet when the toy came out I was like "hey this is pretty good" only to find out later that everyone who apparently did vote for Primal hated it and assumed Hasbro was personally slighting them for "ruining" their fan poll.
To me the Optimus Primal figure gave the figure a surfboard as an homage to TM Primal, and he's a pretty ok "Cybertronian" mode Primal. I donno. I don't get the hate. 
