One of my character's has a PVP discipline spec as the first one in his window but all he does is PVE Shadow DPS. I started out a long time ago with Holy as the main spec but found most folks wanted me to DPS so I just changed the Holy one to Discipline so I didn't have to re-purchase my Shadow glyphs.
You brought in a PUG and asked him to tank the instance for you. I would say you made the right call in that situation though I understand that it harmed your own guild's progression. If you ask someone to come in and tank for you I would say it is unfair to deny them tanking gear that drops. The silver lining is that you kept your word to the Death Knight and there are many raids that wouldn't do that. You might be able to end up with him in your guild, at least my experience suggests he will be positively inclined.