How weird that you should say this. How many Presidents have been defeated seeking a second term v. how many have won. Let's take a look.
Defeated:
1. John Adams, John Q. Adams, Martin Van Buren, Grover Cleveland, B. Harrison, William Taft, Warren Harding, Ford, Carter, George Bush Sr.
Won A Second Term:
Washington, Jefferson, Madison, Monroe, Jackson, Lincoln, Grant, McKinley, Rooselvelt, Wilson, Coolidge, FDR, Truman, Eisenhower, LBJ, Nixon, Reagan, Clinton
That's 18 to 10 in favor of a second term. In the modern era, Eisenhower, LBJ, Nixon, Reagan and Clinton all won a second term while Carter, Bush Sr., and Ford (not a good example) failed to win a second term. That's 5 to 3.
I don't think you'd find a political science class, political commentator, or even talking head who didn't think incumbency was an advantage.