tersignf has a good point that I always overlook, the lefty N, something patterson has going for him is his reverse power lefty numbers, I love having those in a safeco/shea enviroment, but you did say you already have decent lefty power. I guess that what would need to be considered, regardless of opposite manager settings is how many runs is Pierre going to produce for you with his high OBP, speed and avg that can drive in the bottom of the order as opposed to how many runs he's going to cost you with his arm, let's call that his net run production. I still believe that if you have bats that can drive Pierre in then his OBP and speed are waaaaaay more productive than the damage his arm will cause. Safeco has had a tendency in my limited 3 team experience there to hurt OBP quite a bit, which is great for your pitchers, but with a hitter like patterson who already has a pretty low OBP you're essentially going to get homers from him, and even that not in bunches, he only has 1 clean hit hr in his card both vs. L and R and all others are BPHR, and though Safeco is kind to lefties it's still a 13, not a coors-like 19, so I think your offense would suffer quite a bit.
But only you know, since we don't have a link to the team, if what you need is a leadoff hitter then I'd stick with Pierre, If what you need is a 7 hole hitter then I might switch to patterson (still not sure though) every team has a weakness, at least one, my teams lately have many exploitable weaknesses, but ideally your weaknesses won't be your opponent's strength, that's why I refered to the other guys' speed. PBTR, wavygravy2k and bleacher creature are absolutely right, CF arm IS the most important arm.
But anyway...I haven't figured out a way of calculating the effects of the OF arms, but whenever I have a similar situation where I try to see the defensive impact of a player I always match it up with the offensive numbers the player is likely to produce and go from there, the so called net run production refered to above.
By the way, the time I had Pierre in CF, was in Safeco, my pitching was waaay worse than yours, I traded for him after game 57, so he played 105 games for me. He hit .312 with an OBP of .362 stole 51 and scored 84 times and my pitching staff posted a 3.86 era, in case you are wondering who was pitching suffices to say that the combined salaries of my SP was 8.35 M (little bit more than sheets alone) and my pen's added salaries 12.35. The player I traded away for Pierre was Torii Hunter, I honestly didn't see that big of a difference on my pitcher's stats, and Hunter -Perre is a nasty drop. Most likely I was just lucky but it's just my experience. I do have to add that my staff was made up of mostly ground ball pitchers and I had a 1 in both SS and 2B, hence the hit I could take in CF defense.