A sump is not a *requirement* for a successful tank, it mainly makes things easier. A few advantages of a sump are it increases the overall volume of the system, provides additional area to remove wastes (i.e. LR, DSB, macroalgae) that is out of sight. Is a great place to "hide" pumps, skimmers, dosing equip etc. and provides a location to top off etc.
Of course the display tank can function for any of these, so basically I would say that you don't *need* a sump, but it might require a little more work, and not be as clean looking.
Since your tank isn't drilled, if you want a sump you could try some of the siphon overflows like this to set up a sump
http://www.marinedepot.com/ps_ViewIt...ct~CR1511.html