Quote from FE Hoppy:
"Speed increment remains the same because the slats are not increasing CL and the higher pitch for same Alpha doesn't help adding even more pitch for a bit more alpha (Cl) is counter productive as it extends the landing run. [...] I can't really add anymore as I'm just recalling what Emb told me when I asked the question."
I appreciate your problem, but I simply don't understand the explanation you were given. Deploying the slats (assuming they are slats, rather than simple droop) should enable a higher alpha and, consequently, a lower approach speed. That increases the pitch attitude, as you say, which on some types would risk tail-strike. If that's not the issue on the E170, I suggest that the ability to lower the ground-speed on touchdown by, say, 10 - 15 knots would more than compensate for the reduced braking efficiency during the longer de-rotation.
Of course that assessment is at best only empirical, and I stand to be corrected. But it also reflects experience on four other jet types with slats.