Does the one you got come with spare blades?
No, no spares. And honestly I haven't run it enough to the point where the cutters need sharpening or replacing.
Also, each cutter has two cutting edges, so it can be rotated when one side goes dull.

Now... I vaguely recall contacting McJing Tools about replacement cutters (which they don't stock). However, if memory serves me right (could be a big if) they said the Stew-Mac cutters will fit their model. I also think I got some info about this on the ANZ Luthiers Forum (ANZLF).
I'll have to re-research that to be safe...

Would it be appropriate as a jointing tool as well, if you had a dead square drill press platform?
Possibly, but I never thought about it or how it would need to be set up. I reckon a jig can be made for dang near anything if one is so inclined.
My plan is to make a joiner by inverting my router and making a jig for that.

Lastly...

Sorry for highjacking your thread Robin.
Yeah, sorry Robin.
I feel extra bad because we're discussing power tools in a thread that's all about a guitar being built using all hand tools! D'oh!!!