16:9 confusion

[Paul Bravmann]

i about to start shooting a short documentary using a borrowed canon XL-1,
will finish the project with a GL-2. both cameras have a 16:9 option which
i'd like to use--i hope to broadcast or submit the thing to festivals in
widescreen format. BUT i keep reading warnings re: using this in-camera
feature--that it's better to matte the flip-screen or viewfinder (not really
sure how one would go about the latter)  and put the black bands on in
post... what makes this method better? the one thing i do know how to do is
compose a shot, so it's unlike that i'll end up with footage that i'd like
to reframe. so i just don't understand why i wouldn't want to use the 16:9
"effect." would somebody please explain?

much thanks,
phb