As someone who grew up with TT play and physical models, the answer is easy - aesthetics. I'd much rather take the beautiful Marauder than the ugly Orion
I understand what you're saying here, but as I've said in another thread, that's (at least for me) one of the beauties of this game. On tabletop, there were a fair number of mechs I put into my "Meh..." category; same with you, I didn't favor the aesthetics, so I tended to go with others.
Few would argue that the Marauder is a much sexier beast than the Orion, but this game like previous MechWarrior iterations pushed me into using what I had over what I wanted, at least in the beginning. In MW4 the Chimera wasn't that pretty...looked like a cheap Periphery design. I don't much care for the Quickdraw in this game; I swap it out for better as soon as I can. But the point is I'm pushed to use mechs in different tonnage ranges with distinctly different capability and roles, and made get a feel for them all. In my opinion, that's made a better and more rounded BT/MW player.
Other than the obvious opinion rather than objective fact regarding MW4, you bring up that old chestnut of "character". What is the character of the Catapult? We see the missile boat loadouts, the direct fire support loadouts, the one shot area saturation with 8 RL20s, the Artillery mechs with Arrow IV and even an anti-infantry variant with twin Plasma Rifles.
Basically put, even we stick to only variants of mechs quoted in cannon sources, you may as well just have the one chassis with every type of hardpoint. You could make a missile boat Marauder (the -4X and -9M each had an SRM6 in each side torso) and also the Hunchback (the -4J and -4M had LRMs). But if we take the character argument, one should be direct fire support, the other a close combat machine.
As I said above, I'm well aware that the devs won't be getting rid of the hardpoints soon. Not because of any character argument, but largely the practical restraint of modelling all of the possible weapons in each possible location; plus the balancing of the time and cost system. That's not to say it can't/won't happen in future games, but certainly not in this one.
Yes, my opinion, but one that you pretty much lend weight to with your aesthetics argument, and the fact that MW/BT games do tend to be a richer and more varied experienced experience with hardpoints, over the old open slotting.
Character as in (no idea why you felt the need to put it in italics) the battlefield role and general feel of a mech design.
We'll use your two examples...
While there are variants, generally the role and character of mech applies to every design the game produced.
The Catapult is, in my not so humble opinion, one of the best fire support mechs from the early game, which can be nicely improved in the battlefield with a few practical tweaks. While, technically, the Catapult is outdated by the Orion by a century, it looks sleek and clean; a product of advanced mech designing. Everything about it says whoop-ass and looks good doing it. I am biased myself by aesthetics; I prefer using 'walker' style mechs with an avionic styled cockpit, such as the Catapult and its descendants, including the Mad Cat.
But the Catapult backs those good looks up with performance...I also tend to go into the field missile-heavy, to 'Agincourt' the enemy before knocking holes in them with direct fire. I rely heavily on it in my campaigns...at least until I get a hold of the Stalker. Also one of the sexiest walker style beasts in the game...I collectively own more of those two mechs than any others in my multiple campaigns.
By comparison, the Trebuchet is fairly ugly, and was pretty solidly on my 'meh' list until I got pushed into using it, as the best arty support I had in the early campaigns. Came to like using it.
The Marauder just says walking death...it's lean, it's mean, it's scary to see coming at you. I might compare it to a giant scorpion on a rampage.
It may not be the best heavy mech in the galaxy with its primary configuration, but almost always a vicious beast on attack or defense. More than being locked into any specific battlefield role, I'd regard its character as exotic, as the first of a new line of advanced mechs with radical design. It's fairly versatile with its variants, and it's certainly one of the top 5 favorite mechs, based on aesthetics...probably the top 3.
In the case of the Marauder, in some ways I'd say its character overrules its practical battlefield use. It's a rockstar among battlemechs.
Now, with the Orion, the TRO: 3025 describes it as an ancient battlemech design, and it looks the part. I'd say it's one of the earliest mech designs that looks like a FrankenMech, without actually being one. With the Orion I never really equated less style with no performance, as I played it both tabletop and in various MW titled where its role and character is a good all around striker and brawler, quite tanky, and with a diverse mixed loadout that has seen a lot of configurations over the years.
This is not to say mechs should never be taken out of character, because I myself have tried a basic Catapult loadout that removes the LRM racks for SRM's instead, to make a jumpy Kintaro...still, we are limited by the hardpoints of the mech. We can widen that out with more variants; something I totally support.