What do you mean by equivalent? If you are comparing 3rd party "pro" ranges to "standard" Canon & Nikon, yes I agree, but also the price difference is not so great these days. If you are comparing to the top line C & N, I think there are a significant number of differences - can't speak for Nikon in detail, but Canon "L" series will show:
1 - optical superiority - visibly sharper and better contrast, in part due to superior coatings.
2 - build - tougher construction and better whether proofing
3 - performance - 3rd party lens use a copy of the mount and lens/camera interface, they do not work form the Canon specs. This usually means Canon lenses tend to work better on Canon bodies, and in particular the AF
is faster and more reliable. In fact there are situations where older 3rd party lenses will not work fully on newer Canon bodies.
Ultimately you pays your money and takes your choice. There is a difference, and significantly more than 1%. Whether or not that difference is worthwhile is up to the individual. Prior to switching to EOS, I used a mix of 3rd party and Canon lenses. I had good 3rd parties and bad - each had their own quirks. When I changed to EOS, I decided to stick Canon - each lens works as well as the other, provides consistent results and I don't have to worry about each lenses quirks ... whether or not they are better lenses, ease of use and standardisation makes for better photography.
Colin K. Work, Pixstel