pg_dumpall forces plain text format

I was trying to do a data+lo dump using pg_dumpall:

pg_dumpall -C -a -b -o -Ft > dumptest.data

Unfortunately I kept getting this error:

pg_dump: large object output is not supported for plain text dump files.
pg_dump: (Use a different output format.)

Upon looking at the pg_dumpall shell script I found out why:

PGDUMP="${PGPATH}/pg_dump $connectopts $pgdumpextraopts -Fp"

Is there a particular reason why plain text is forced? Could this be removed
in the next version? Or even moved to a pg_dumpall command line option? I
really dislike using nonstandard tools.

Regards,
Andrew