Even if they have a similar focal length, there is still aperture. And even if aperture is similar, there is image quality. Resolution, CA, distortion, vignetting, bokeh, contrast, colours... So not only will you change lenses for 2mm, you will also change it for "different-looking image." Or you might choose to pack a different lens because it is 100g lighter and 2cm shorter! Or WR, or silent focusing! Many things to consider!
Primes might not make sense in theory, but they make sense in practice. You have to try them to see!
That being said, zoom lenses have come a long way and are now serious competition against primes. Especially when looking at ones like the Sigma 18-35mm f1.8, and the previous Sigma 8-16mm. These give primes a run for their money! But they are
big!