The Shivans seem to be uniquely adapted for close-quarters null/low-gee combat, so if I were to make a guess...I'd say they're really not interested in anything but protecting their own ships from boarding action. They can't count on being the correct size (I mean, seriously, a Shivan takes up three or four times the space of a human) to move around aboard an enemy ship or on their enemies not having set their ships up to provide a range advantage, so I would expect them to have been somewhat smaller and with more adaptable armament if they intended to take a fight to somebody in person.
The Shivans are also living in cybernetic bodies. We know they're fairly strong, we watched one smash a human into a wall hard enough to be apparently disabling despite body armor, but that was in a microgravity environment. They may not function well in a one-gee environment, if for no other reason then their unfamilarity...and I personally doubt they'll be able to run along the ceiling in one.
