I don't know your hairdressers reasons, but I personally dislike doing that sort of color for several reasons.
1. if there is any color on your hair at all, and yes the color from 6 months ago counts, it's an ordeal to make that color even. Sometimes, depending on what has been used, it's not even possible to lift up out of the ends.
2. NO ONE ever touches it up as often as they should, the further out your roots get, the worse it is for us to match it up to the blonde on your ends. Your root will get very light, and further out, stay golden, leaving a lovely line that we then have to get rid of.
3. If you have existing highlights, mixed with your natural color, by the time your natural color lifts up to the color you want it, those highlights are already starting to melt at the ends.
Honestly though, my main reason is reason number 2. I *hate* seeing that color walk around with roots. And roots start showing at about 2 weeks. It's a high maintenance color.