Mouse Keys

We mean of course the buttons on the keyboard you use with the mouse. The buttons on the mouse don't actually matter since there's only 2 or 3 of them and you can't use the modifier keys to multiply them because that splits the user's attention between the mouse and the keyboard. Besides, when the mouse keys are used to good effect then the buttons on the mouse are only needed in very special circumstances. These are dealt with below.

(The mouse wheel matters much more than the buttons on the mouse, but not for its intended purpose. Scrolling could be just as easily performed by pageup/pagedown if only focus rules weren't so application-centric. Of course, there would remain the problem that pageup/pagedown are situated for left-handed people for some reason.)

Discover the real mouse buttons

The mouse keys are the 12 keys and 18 to 27 key combinations directly under the off hand (the one not using the mouse).

QWE ASD shift ZXC Alt Space

Using ctrl in key combinations is disrecommended since it contorts the pinkie every time instead of just 1/3rd of the time. The ctrl key would be in a better position if it were in the place of the alt key (under ZX) and the alt key moved right (under CV) taking space away from a shortened space bar. As it is, using shift-alt-key is easier than ctrl-key. Keyboards with a "Windows" meta-key could also use that.

Since the right-hand modifier keys only exist for left-handed people, if manufacturers were willing to make left-handed and right-handed keyboards they could remove these keys in the right-handed version. The empty space would be given over to the space bar to make up for its shortening on the other side. Not that this is necessary since the space bar is unnecessarily long anyways.

Continuous operations and mouse QuasiModes

The only times the buttons on the mouse matter is when it's a continuous operation that requires they stay pressed down for an extended period of time. When the release of the button exits or triggers the operation we call such an operation a quasimode. There are much fewer such operations when the mouse buttons are used to good effect. In particular,

drag and drop, is replaced by pick up and drop
drag selection, is replaced either by 'drop bound1 - drop bound2 - select' OR 'drop bound - drop bound - select' OR 'drop bound - drop bound & select' depending on the choice of flexibility versus efficiency required (eg, the last choice can be performed with two clicks of a single button)

Only the "paint" QuasiMode in paint programs remains a continuous operation.

In order to not suffer from remaining a mouse quasimode, a continuous operation has to demonstrate strict on-off-on-off order that can't be rearranged even in principle. This is not the case with drag and drop where it's incredibly useful to pick up multiple objects before dropping them either one at a time or all at once. In PUI systems, this has forced the invention of 'selection' as a separate phase, leading to select-drag-drop. It is also not the case with drag selection where it would often be useful to (re)drop the first bound after the second bound.

In order to actually benefit from being a mouse quasimode, the switches have to be frequent enough that the user can't afford to take their attention away from the mouse pointer during an extended operation OR the user's attention must necessarily be on the mouse pointer either at the start or the end of the operation. In this case, the benefit comes from keeping the user's attention focused on the screen and the mouse instead of splitting it between the mouse and keyboard. This is precisely the case with the paint quasimode in the paint program. Neither DragAndDrop nor DragSelection? require such precise targeting that they benefit from being mouse QuasiModes.