You can wear leather or chain for stats when grouping. When you're soloing you need the mitigation. In raiding, besides mitigation you also need resists or you'll get squished. I actually just tend to keep my base str around 400-425 now, because most groups will get that buffed up to close to 500. The additional sta and resists lets you survive quite a bit more in grouping than you would think. Peeling is less of an issue if you can survive a bit to de-aggro, so it lets you go harder. (Many groups don't have good MTs or de-aggro skills)