Firing in all directions = getting yourself surrounded. Seldomly a good idea
While Skymarshall isn't ideal, it allows to concentrate fire while providing cover. Disembarking is faster, you can start real action by turn 1-2, instead of turn 3 (you can't get these soldiers any faster under the Skyranger's belly, especially since you're forced to skip turn 1). If you happen to land in such a way that either side of Skymarshall is more or less clear, you can disembark a good deal of soldiers by turn 1, and start to flank the enemies on the other side by turn 2. Such a thing is impossible with Skyranger - you're completely exposed from all directions.