That would be a lot better than the current system. As it stands a candidate must be invited, and the only way to be invited is to pull 15%.
1976: Not much change really. Might butterfly the gaffe that Ford and Carter both later credited with deciding the election. Ford might just pull it out.
1980: Carter refused to be a part of any debate with Anderson. As a result, Reagan and Anderson debated each other, then Reagan debated Carter. No change, as Carter is still destroyed. Anderson might win a state or two.
1984: Don't know enough to comment. No change I can see.
1988: Ron Paul might get some real traction.
1992: Throw the libertarians in and you have some real fireworks. Could go either way. Clinton or Bush.
1996: Perot would crack 10%. The Reform Party probably wouldn't implode like it did in OTL.
2000: Nader cracks 5% and gets his federal funds. Bush wins fair and square, avoiding the Florida fiasco.
2004: See above.