I think it's stupid to have the longsword as your backstabbing weapon of choice. A dagger should have a huge backstab multiplier, while being inferior in reach and base damage compared to the longsword.
So if your sneak attempt fails, you are left with a slightly inferior weapon in frontal combat, but a successful sneak attack will yield much better results. The situation in oblivion is that you're always good with a longsword - stealth or frontal combat. I won't even go into the Morrowind 2-handed stealth attacks..