Rather ineffective compared to military transport planes, that's for sure, but they're still useful enough that it's very likely we would see their use in a china invades taiwan scenario. Provided certain prerequisites are met, of course: Chinese would have to capture and secure airports for those airliners to land on, they would have to secure the machinery needed to help disembark the troops and cargo, they'd have to control the skies pretty much 100% as such an airliner would be a sitting duck. Though, i have to say, their speed isn't that bad, a b737 or a320 is even a tiny bit faster than a an70, let alone any prop powered plane chinese have.
Alongside the troops being transported in the upper deck, the lower deck has additional cargo space which could be used for transport of smaller to medium size boxes of cargo. Of course, to empty up such a plane would take longer than to empty a military transport plane with the backdoor ramp. Still, all these annoyances being listed, if you have a secured airport/sky, you can bet those planes will be used, you'd have to be crazy not to take advantage of them.