Like others have said why not?
I use my 40" for gaming and watching videos from my PC.
And its currently my only monitor since my last one decided to die, picking up the u2311h when it goes on sale.
so your gonna have a 720p tv which is significantly less pixels than a 1920x1080 monitor and its gonna be stretched out over 36" or 42" or however big it is
the picture quality is nowhere near as sharp because less pixels per inch
tvs also may suffer from input lag and some other factors which make them undesirable to use as a monitor
monitor lcd's are far greater than tv lcd's