I had this done actually. Despite Mwamba telling me I could only get 40% out of my donor, he ended up taking 60% because the other 20% was replaced with beard hair.
Main rationale is that my beard hair is 3x times thicker in caliber than scalp hair, and I would rather risk donor look less natural than scalp. When kept short, beard hair vs scalp hair is indistuingable, but for very long hair, beard hair can be very noticeable, also, beard heard tends to become grey faster with age and i'd also rather have grey hair on donor and not on top.
Personally, I would only have scalp hair on my top. But if you think your beard hair is very similar to scalp hair going the direct route also makes sense.