If the IRS is not going after Romney for tax issues, then there is no reason why the public should be concerned about his tax issues. Reid is just fishing for something that may look illegal and controversial, but is actually perfectly legal.
It may be legal (though only strictly speaking since he is taking advantage of legal loopholes), but it is also controversial.
Also, we have no proof that Romney didn't do anything illegal...
The burden of proof is on the accusers, not the accused. "Innocent until proven guilty" and all that other far right nonsense.