That argument doesn't hold up very well. Why is an SLR better if it has a tiny viewfinder or screen? Sensor and optics do matter. That said, people may prefer the retina display or carrying only one device (the tablet).
You don't use the screen while shooting on an SLR camera - you look through the 'tiny' viewfinder. The difference is, you're not looking at a 1cm wide display, you're looking through it and seeing exactly what the camera sees. It's effectively "infinite resolution" and actually quite "large" to look at compared to an actual screen.
The point is simply that no matter how good the optics and sensor are, if your shot is out of focus it doesn't help, and if your composition is bad it doesn't help. A larger screen helps both, and an SLR viewfinder is even better.