Nothing wrong with replacing the boot only. Places often recommend changing the axle because 1) not sure if the CV joint is damaged, and 2) cost of aftermarket axle is same as boot + extra labor to change the boot.

I repaired/replaced boots in the past (but not on the X5 yet) with mixed results. The time and mess involved was not worth it. It later leaked so slightly because the part did not fit well. Now I just buy whole assemblies.

If you change only the boot and later find the joint is damaged, or the other side boot rips, you have to go back in there and do it again. If I had 90k on original axles, I'd tend to replace them (but that's me.)