I know this is an ancient post but I stumbled upon it in a Google search and figured I could add a bit more historical context.
As others have mentioned (and linked), Skullclamp was proven pernicious in the Standard and Extended formats of its day. There was actually a period where it seemed it was getting banned from a new format every B&R announcement (first Standard, then Extended, then Magic Online-only formats).
That said, nobody knows for certain what impact it would have on Legacy or Modern because it was never legal in either format. Both of those formats are relatively new, and unusual in that they're popular tournament formats that are younger than (most of) their card pools. Because both formats started with a large card base, both formats also started with banlists that are somewhat speculative. It's tricky business because if they started with nothing banned, both formats would have been ruled by known broken decks for quite some time (what Zvi Mowshowitz termed "Oh Lord, Not Again"), so Wizards extrapolated from past tournaments in other, similar formats.
The result is that both formats have a number of cards that were "preemptively" banned in this fashion. Sometimes those cards get unbanned; Mind over Matter, Replenish, and Land Tax are apparently not as bad as they were in their original contexts. I doubt the same thing can be said for Jace, The Mind Sculptor/Stoneforge Mystic in Modern, or Mind's Desire/Yawgmoth's Bargain in Legacy.
While Skullclamp was never proven broken in current formats as such, it ruined Standard, eventually proved broken in Extended and was almost Vintage playable. It's also arguably a more resilient Glimpse of Nature (and certainly has a similar effect in the same decks), and that card is also illegal in Modern. It's almost certainly staying banned forever in Modern, and deservedly so, since that format is intentionally kept at a power level at or below the old Extended formats where we already established Skullclamp was too good. Legacy is fast enough that I can see an argument for unbanning it coming up at some point in the far future, but there's a danger of it becoming an automatic 4-of in every aggro deck and becoming an autowin against control. There's also the risk that it fits too well in a creature-based combo deck like Elves!, or that it combos too well with some other cards printed in the many years since it was ever legal in a non-Vintage format (Stoneforge Mystic? Puresteel Paladin? Scars block in general?) So I don't like its chances there either.
[^1] Technically there was one Magic Online "Modern" tournament, which I believe was open only to employees, where Jace was legal. But then the standard bannings happened and by the time the Modern banlist was finalized for the general population Jace was on it.
EDIT: After some more googling I dug up the development articles from when the banlists were new: Aaron Forsythe on Legacy and Tom LaPille on Modern. It's telling that neither felt the need to go into much detail justifying the banning. Its effect on Standard and Extended really was that bad.