That is fair. I think he is having a pretty good season too. So far, his HR/FB rate is a at a career high and likely a anomaly so his other peripherals may look better by seasons end.
The biggest concern is those 200 IP since 2007. He could certainly be a exception like Burly! but my fear is him breaking down and the team having a large contract limiting the Jays from keeping other players. It feels like a RA Dickey type move as you'll have to give up at least a Norris and Pompey to get him with a Reyes-like player and decline heading into the final 2 seasons.
If this team also isn't inclined to taking on a large contract, I would still keep the prospects, aim for someone cheaper to help the push into the offseason and spend the $20 on a UFA like a Samardzija, Price, Zimmermann, Kazmir, Fister, Cueto, Kennedy, Iwakuma, etc.
To me the cost of Shields, the future of Shields (my own bias opinion ) and the other opportunities available in UFA make him a pass.
E: Also, I know there is no promise a UFA would come here. So if Shields is still on the table and you miss out on the UFAs trade for him or someone else using Norris/Pompey in the offseason.