Let's get a couple things straight.
1) It is called dethatching ..... not thatching
2) Even if you double passed with a vertimower you will still be able to rake stuff up, not to mention a double pass will tear the shiit out of any lawn
3) Thatch is a problem primarily in turfgrasses that spread via rhizomes and secondarily by stolons. Those that exhibit both stand the highest risk of producing significant thatch. One inch of thatch is the upper threshold where it needs to be addressed IMO.
4) Kentucky bluegrass is the only cool season grass that is likely to produce significant thatch, and therefore is the only one that really needs to be monitored. All this talk about thatch and cool season grasses without specifying the type of grass is idiotic.