You need insane amounts of Subtle Blow for it to have any real beneficial effect on whatever youre fighting. So -6 isn't going to put you into any more danger than normal.
The Subtle Blow REALLY is not much of an issue, if you have Rajas, thats +5, and only -1, not really a big deal at all, and 4STR > 6 Atk so I'll take the -1.
Yes, this does have the same Haste as Dusk+, however, you fail to see the big downside.
Adding negative points to Subtle Blow is essentially feeding even more TP. While that wouldn't be a big problem in alot of situations, its a big no-no for things like NMs with devestating moves or parties where your healer already has a hard time keeping up.