How many people train in flats?
Reputable coaches like Jack Daniels recommend using racing shoes for some training, and most of the people I know who train in flats also use them for speed work.
Using flats for base work is unusual, but there is a sort of local "cult" at letsrun who are into it.
It's important to understand that one doesn't need to wear the same type of shoe for every workout.
Regarding trails, it's not an issue for me because I don't do speed work on trails. But if I were doing speed work on trails in prep for a XC race, I'd use the same footwear I'd use in that race (probably XC spikes).
The tiger paw is a really good flat to start out with. It's well cushioned, but still has a low heel, and no medial posts or stability "features".