The Hardrock is an entry level blend of a cross country and freeride bike. Of all the entry level bikes out there, the Hardrock is renowned as having the toughest frame.
The need for disc brakes comes from riding through snotty terrain. However, riding through sandy terrain can also greatly accelerate the wear on rims. If you do decide to buy discs, prepare to spend at least $600 on the bike as lower level disc models sacrifice on other components to get the discs in. The Hardrock Comp disc is decently equipped. It could use Deore shifters instead of Alivios.
For the cost of upgrading Shimano shifters and levers, you could probably just switch the thing over to SRAM/Avid.