I think it's the default advice to do all 4 of anything anywhere near the wheels when you are dealing with an all-wheel drive vehicle. For some things I can see why, especially on a newer car, but other things, not as much. I have been told so many times that I should do all 4 of something on my CR-V by one mechanic, and then told it was not necessary by another. Like brakes. I have not had all four of my brakes done at once yet. I think struts are in that same category - I would not think it's necessary to do them all at once.