First, I'm really glad that PC/JS are at least flying him in. It makes sense to at least give him a workout and a fair shake after being such a class act for us the first time around. I hope they sign him and give him a real shot at the roster, but just giving him a chance is a nice start. He deserves it.
Second, Seattle might conceivably keep Quinn and Johnson even with the addition of T-Jack. We don't have 3 QBs fighting for "starter" reps this offseason and there should be enough reps to figure out who our #2 QB should be even if there are 3 guys vying for it. It would probably be better if they kept it to a 2 man competition, but if that happened I'm guessing they keep Quinn and axe Johnson, which I wouldn't care for much.
I hope they sign him. It really did seem that T-Jack was starting to gel at the end here, and don't forget that Seattle actually entered the 2012 league year banking on T-Jack as our 2012 starter (Flynn was unexpected, and Wilson was not a sure thing until we secured him in the draft). He's not a great starter, but he's not that different from guys like Alex Smith and Colin Kaepernick. If Harbaugh can make those guys productive, I don't see why Pete couldn't do it with the best roster in the league.
Mostly though, I just hope we sign T-Jack because I don't feel great about banking on a sub 5'11" mobile QB to stay healthy while rushing for 500-1000 yards this season. Wilson is a bulky QB for his height and knows how to preserve his body, but it's just one of those things, he's a higher risk for injury than most QBs. Therefore it's that much more important to have a backup who can at least keep the team from being a disaster. With T-Jack, he may not be pro-bowl material, but we know what we are getting, and it's good enough to win games on this roster.