I won't refute that the addition of more (moving) parts is the addition of a point of physical failure, but I should chip in with my experience using a slide-out for three years without incident. I aged out the battery, put it through standard klutz drops, probably got it wet a couple times, the camera is smeary/dusty to the point of oblivion, the OS (froyo) started acting weird and hiccuping with microSD content, and I painted the whole thing because I hated the original color (great deal on price though).
No problems with the slideout or the keys, and I played the ever-loving fuck out of them using emulators and a simfile rhythm'er (Beats).
OTOH, the side buttons aged, notably the volumes were getting barely responsive. Took it apart towards the end of life, no dice, guess the fittings were wearing out past tolerance.