Wanted to say thanks to the devs for adding this feature, but also mention a warning....
Multi-selecting components in the tree that have many nested components can cause the program to take a LONG time to respond. In some cases, it can be so long that it almost feels like the program has frozen.
For example, I have a calendar I've created, and each day is a column with anywhere from 3-5 paragraph components in it. Multi-selecting 10 days (columns) at once is tantamount to selecting 30-50 paragraph components, and it can take 10-15 seconds for the program to respond. Deselecting results in a similar (though slightly) shorter delay.
I don't know how feasible it is to create a means of instantly canceling a multi-selection that one might realize is going to result in an incredibly long delay. For example, in the old days of Photoshop, certain operations could cause the computer to sit and "stew" for 10-15 minutes while the program was doing all its math calculations. This was in the old days, when software was outpacing hardware. Now it's largely the opposite, but if you're working with a 3 gigabyte image with 50 layers and decide to rotate the whole page, it could cause the program "lock up" for minutes at a time, so Photoshop added an "emergency escape" from this scenario.
Hitting the Escape key instantly cancels the last executed operation. It might not be a bad idea for the devs to consider something like this for BSS's multi-select feature. Hitting Escape would just instantly de-select everything and give you back program control.