Intersect Tool Missing! Available as separate plugin.

[micheus]

I also think this way.
User select one or more objs (A) + select operation + select one or more objs (B) + execute.
By selecting many objs, user is deciding for make their union before proced with the operation:
A=A1 U A2 U A3 U A4
B=B1 U B2

at the end, you'll have the regular operation, p.ex.: A U B

it seems logical for me.