It's a big deal when maintaining a fork. It's tempting to merge upstream commits back into your branch, but you should always rebase and keep a clean patch set in such situation.
Of course if you're planning a hard fork, merges may be unavoidable. But I've seen too many Franken-linux-kernels which were forked from 4.x with periodic merges whose correctness is impossible to verify. Inconsistencies eventually build up with each merge.