I dont know this lens ...should be versatile for some outside daylight stuff..but adapters seem to get bad vignetting on small rear apperture lenses(imo)
3.5 is slow, but even if you have fast primes you're not gonna shoot f1.4 or f2.0 often. Most movies are shoot at >= f2.8 anyway. f4.0 is very common.
Fast primes are good for night exteriors when you don't care about bokeh and what's out of focus simply because there's usually not much to be seen in the background - they allow you to save $ on lighting.
I have the lens you mentioned and I like it a lot. It's sharp, has constant aperture within entire range and is well manufactured. It's one of the best FD zooms available. Combined with 80-200 f4.0 L (cheap) and 20-35 f3.5 L or aspherical (not so cheap) it can cover the most common range of focal lengths.
Personally, I think you should have both zooms and primes. FD lenses are cheap. Check KEH.com - I bought some great lenses from them.