I've used three physical keyboard phones in the past, one clamshell and two sliders. The slider was my favorite form factor.
Let's try out one of my old parallel slider phones, shall we? How about the LG Enlighten, it's the first that fell to hand.
GRIP: When the phone is turned horizontally and slid open, your index finger(s) tend to rest on the edge of the keyboard half and on the back of the screen half, quite securely gripping the phone. I'm concerned that having the screen angled up would make the phone more difficult to hold as it would no longer rest on your index knuckles.