People use live rock cause it's the best product for the job. It's a naturally occuring substrate for the tank that replicates the reef perfectly since that's where it came from. But do you HAVE to use it....no. Marco dry rock works just as good in the long run. IN fact, most of my tanks have at least 1/2 live rock while the rest was dry rock (not base rock, just dry/dead rock) It was arqonite based which you should aim for, don'treally want just any ol' rock.
Caribsea puts out a dry rock, "reef rock" and "reef bones" (base rock and porous rock respectively) that works wonders.
Give any appropriate rock, weather dead or live sufficient time and it'll be considered "live'.