There is no way to tell pg_basebackup to "just compress the thing in a
default way". You have to pick some number and then write -Z8 or
something. I suppose that interface was copied from pg_dump, but there
it's not that commonly used because the right formats are compressed by
default.
I suggest we add an argument-less option -z that means "compress", and
then -Z can be relegated to choosing the compression level.