You could and it does make it even simpler (one button does it all) but I recommend against it. You would have to trust the undercut value. Because it also does a scan for the item when you alt click you can see that quite often there will be someone artificially lowering the item price in hopes to drop the price and buy it cheaper. Say some herb usually sells in stacks of 20 but the price for a single unit is 40g, which can go unnoticed (if no add-ons you'll only see the full price of the stack, not by unit) . Someone can easily pick one unit of that herb and post it at 20g or less.
The leveling experience for the Burning Crusade and Wrath of the Lich King levels of 60 to 80 has been greatly reduced from 6,689,030 XP to 4,412,600 XP which is a reduction of 34%. This has been a notoriously lengthy leveling section, and this reduction will be very noticeable and appreciated by those players leveling new characters such as those allied races!