A PBS cube will combine two beams that have opposite polarization. That is, if you have one beam that is vertically polarized, and another that is horizontally polarized, the cube will combine the two beams into a single output beam of roughly the same diameter as the input beams, but with roughly twice the output power - assuming both input beams are the same size and power to start with. (You loose a few percent power as the beams pass through the cube, and the optical face inside the cube can give you a little splash, which may increase the beam diameter slightly.) Of course, the output beam will have random polarization.
Also note that, while a PBS cube is normally used to mix two beams of the same wavelength, it can also combine two beams of different wavelengths. Now, I know you're asking, "Why would you want to do this with an expensive PBS cube when a cheap dichro would work just as well?" Here's an example:
Say you had a pair of red lasers, one at 660 nm and one at 650 nm, that you wanted to combine. You'd be hard pressed to find a dichro that would combine two lasers that are so close together in the spectrum - even if you had several hundred dollars to spend. But a PBS cube will work just fine, so long as both lasers are polarized. You simply rotate the mounts on the lasers so one beam is horizontally polarized and the other is vertically polarized, and you're all set!
Adam