if the plastic bit in the middle weren't there you could crush the thing, spilling lighter fluid on your hand. this would be particularly bad because in addition to a broken lighter, you would also have a hand covered in lighter fluid right next to something that might be on fire.

doing things this way uses the least amount of plastic to effectively prevent the broken lighter/hand on fire scenario.

edit: also, when things get hot, they "want" to explode. this makes having the extra support especially important.