I guess if all you carry is a multi-tool, CO2 inflator, and tube, and don’t mind them getting wet and dirty, it is an perfectly OK solution.

And I do agree that saddle bags flop around to much when mountain biking. But I take a lot more than the above three items with me on the mtb, so use a pack.

And on my road/gravel bike, the bag bouncing around it not an issue.

Also, even when I have used a small sadle bag with an MTB, it did not interfere with me getting behind the saddle.

But I am a total Fred.