postmaster.opts, moving datadir around, and pg_ctl

I had datadir in /dir1 and I started postmaster with:

postmaster -D /dir1

this recorded "-D /dir1" in /dir1/postmaster.opts. Then I stopped
postmaster and moved /dir1 to /dir2. I then started postmaster with:

PG_DATA=/dir2 pg_ctl start

It worked normally. But:

PG_DATA=/dir2 pg_ctl restart

failed when starting postmaster because it used datadir from
postmaster.opts (/dir1). This worked though:

pg_ctl restart -D /dir2

So there's a slight inconsistency between the start and restart command.

Would it be nicer if postmaster doesn't record -D in postmaster.opts, or
if pg_ctl strips it? Or is it entirely my fault for not adjusting
postmaster.opts when I move datadir around? IMO it would be nice if
datadir is portable (i.e. it doesn't contain its own full path).

Also pardon my ignorance, but what is the purpose of having
postmaster.opts inside datadir?

This is postgresql 7.4.3 on Linux.

--
dave