Its not a matter of location damage factors for gameplay reasons. Its one of those "the design does not look solid enough or safe enough" feelings for me.
You can go #energyshields but to me visually the design looks flimsy and not something I would want to take into battle. It relies too much on Rule of Cool to help with its suspension of disbelief.
But if among the most basic early when-it-gets-implemented cosmetics is an armor plating structure to reinforce those areas I can deal with it. If someone wants to see the mech looking flimsy they can have their flimsy mech.
But if I was a reaper I would take that thing to the nearest omniframe engineer and tell them to reinforce the fuck out of that cause I do not trust an energy shield alone to keep me alive or keep my mech from falling apart.
I think "its open because of energy shields" excuse comes from combination of a lot of decisions that kinda snowball on each other and were created by the concept that they want to go with.
Till today everyone thought we gonna play with a character like in Firefall, we gonna have different armor/frame/suit but it still would be a character that we could customize and run around with. But then we find out that we are going to play World of Mecha. Meaning from the looks of it we will be inside of a
vehicle most of the gameplay time. Most people here probably were not looking for a game where you are a robot. So they had to compromise and strip out the armor just so they can say "No no, hey look, there you are, you are the character right there in that suit!". Which I guess works for the purpose, but it messed up what mechas are. Do a quick google search, see how many armored suits you will see with some force field in front... not many at all (if any, I couldn't find any), because when you think mecha, armored suit, you thinking armor and guns, not a dude with pink hair and yellow jumpsuit staring at you in the middle of it. That's why a lot of people right away have a problem with the way it looks, and the "it has a energy shield" excuse feels kinda forced and weak.
Basically they just need to separate character from the mecha, and make the mecha a vehicle. Sure we can be inside of that vehicle most of the gameplay, and I don't have a problem with that, but gee don't butcher the vehicle just because you feel like people have to see their pretty hats, ha!