Responsive means you wish to adjust the width of elements based on the width of the screen.
The current implementation of responsiveness in Mendix is only useful for downsizing a screen... But responsiveness is meant for larger resolutions too.
Meaning sometimes at lower resolutions I need to make parts of a screen wider than they are on larger screens because otherwise the content of that column does not fit.
If then the default column weight overwrites mine, then I can't control it for medium width screens.
So if Mendix wishes to have a default width for columns it should check whether custom classes were added on these columns or whether it should use the weight set in the modeler.
Or instead of allowing custom classes it should have weight attributes for all screen sizes and also variables to set the trigger points in pixels for the responsiveness to kick in.
With the new Layout Grid, you shouldn't really need to use the custom bootstrap css classes anymore. Instead, you can set the weight of each Layout Grid column directly in the properties of that column.