Importance of using a great Flash/Speedlite in photography?

[conquerer]

I am shooting a Canon 40d. close lens are 28-135 IS and Tokina 12-24/f4. I want a closer Zoom or Prime...say the Tamron 17-50 or Canon 35mm/f2. My dilemma is this: I could get a cheap prime/Canon 580 EXii speedlite for the same price as a good 2.8 Zoom. This would seem a no-brainer to most people, but I don't take many shots of people. Is a flash mostly used for portrait-type shots? Shoudl I get the cheaper lens and flash or good zoom? Give me your reasons