I have never really thought about it until recently.
I can see that a baseball card signed by the player might be worth more than an unsigned one.
Over in New Zealand the $5 note has Sir Edmund Hillary, the first man to climb Mount Everest, pictured on it.
He still lives in NZ and apparently gets asked to sign quite a few when he is down the pub.
That would be an interesting note to own as the signature belongs to the person depicted.
But a picture of a Jace should be signed by Jace!
I can see that a signature raises a piece of mass-produced art above the level of the other identical pieces and serves as a sort of link between the artist and the copy of their work.
I can also see that the thrill of hunting down difficult to find artists would add a new element to collecting.
I passed on an Alpha booster for 699 Euro because it had Richard Garfield's signature and his drawing of a dragon on it. I am kicking myself for not getting it at that price but the booster was in a real mess, not even the signature could redeem it!
I wouldn't mind a booster with the WotC team on them but you certainly wouldn't want the full set done, after 2500+ boosters their arms would fall off.
I think just a Revised Booster (when MtG started for me) with the creative team signatures would suffice but the logistics behind that would give you a headache and it probably wouldn't mean anything to anyone else.