In my humble opinion, a good SF is ... the icing on the cake. A cake can be "ok"... even "pretty good" without icing - but it'll never be great.
Good guards/Bigs are the cake itself - you need them to have any cake at all. But no SF? No "great cake".
So... assuming a team is solid-to-very-good in 1 area of Bigs/Guards, and "very good" in the other, then a great SF will make the team excellent. And I firmly believe that "no great SF" on your team means "no great team".
http://with-malice.com/ - The half-crazed ramblings of a Lakers fanatic in Japan