Note also that the Grand Jury is a check on executive power: that's its function-- to make sure that there's merit (and not just an abuse of power) before we charge people. Now, of course, it doesn't really function that way these days.
The grand jury also serves as an investigative/inquisitorial body: it roots out crime, and has almost no limits in what it can pursue.
It can't really fulfil these functions (esp. without a judge at the helm) if the rules of evidence were in the way. And the rules of evidence, more importantly, would interfere with its role as the preliminary conscience of the community: they can consider the character of individuals, for example, when deciding not to charge.
And finally, when we're dealing with a sophisticated criminal enterprise, we don't always have a great deal of information at the start.
Great point from Ryan: the rules also maybe enhance the legitimacy of the justice system: the jury trial is the most visible aspect of the system, and the rules foster faith in the basic fairness of the process.