Unlike most of the FA starters, Wheeler has the upside to transform into a top 10 pitcher (ala G.Cole). Chances of that happening are (always) relatively low, but they aren't anywhere near zero. Guys like Keuchel, Porcello, Hamels, Bailey, Roark, etc. don't have that upside. On a team that needs top of the rotation arms (v. depth) that has to interest us (especially given his age).
Regarding the durability - he's been healthy the past 2 years. Every pitcher is a ticking time bomb. Guys like Ryu and Strasburg must have as much, if not more risk that Wheeler. With the depth we have, I'm all for getting Wheeler and taking a risk on Jimmy Nelson, T.Walker and even Pineda (although he'll cost most). Guys who also have big upside (but also huge risk). We already have a lot of steady eddies who don't move the needle much.