I agree. Especially multihit moves can't really be avoided with the whistle (such as Sheik Upair, Fox Upair, Greninja Upair, Ryu Upair,....) because of the tight framewindow if all the hits connect. So avoiding killmoves with it is kind of risky as well as the timing. If the enemy predicts the whistle or tries to wait for an airdodge, he'll probably also mess up your timing for the whistle. It's really situational.
The whistle isn't completely useless, but still it's tricky to really find situations, where it's better to whistle then to jump out. Whistle can be a nice option instead of an airdodge close to the ground to avoid the lag you would suffer by landing with an airdodge on the ground. By whistling instead, you avoid the attack and you can act right away on the ground. Offstage I think it could be useful to avoid a spike perhaps, but you won't get a great advantage of it neither and failing with the timing would be crucial.