I did the White Mountains Presidential traverse and the Pemi trails–which are not the easiest trails in the US–this late summer in a pair of Salomon Speedcross. I had never hiked with trail runners and never tried either trail, so everything was new. By way of comparison, I usually use a mid-weight hiking boot (La Sportiva Omega GTX). I had a 35lbs pack that steadily reduced in weight over ten days. Here are my observations:
Pros: My feet felt much better at the end of each day; my knees felt a lot stronger at the end of the hike than usual.(I have a torn patellar ligament in one knee and scar tissue from a fracture in the other, so this is the ultimate litmus test for me). In general I was less tired; no bruising, aching, damaged toe nails, feeling of stepping on hard cement, etc., which was especially remarkable because the shoes were brand new and my hiking partner was aching the entire time–he wore lightweight hiking boots. I only came close to twisting an ankle once–less then usual, to be honest–and it wasn't bad.
Cons: when your step lands in the space between two rocks or on a very uneven surface, your foot will bend, fold, contort, and fit itself to the contours of the trail, and this is painful; the traction on the Speedcross, despite its apparently aggressive sole, was rather slippery and got worse as the days went on and the treads wore down; in general, one must remain more thoughtful with respect to foot placement (but isn't that a good thing, in the end? At least it's better than feeling like you're walking on hammers). The shoes wore down more quickly than the boots. I've had the Omega's for two years and I feel like they're just broken in; the Speedcross have one more serious hike left in them.
In the end, I would probably upgrade to a lightweight hiking shoe or a more robust trail running shoe for the Presidentials (though I got the Speedcross because it advertises itself as aggressive). But given the choice between trail running and hiking boot, I cannot imagine ever going back. The loss of weight on the foot seems to counterbalance the loss of stability. If you're really worried about ankles, why not try a lightweight ankle brace instead. That way you can remove it if you discover it's unnecessary.