Romney's tax returns are relevant in that he has proposed a tax plan which includes yet more tax cuts for the wealthiest citizens, and opposed any measures that might include in any way increasing their taxes or eliminating tax shelters (the latter, ironically enough, is a position his running mate supports). If, as appears highly likely, he has paid few or no income taxes in recent years, it will really undercut the proposition he is advancing - that the wealthiest need additional tax relief in the larger best interests of the economy. In addition, to the extent he has participated in offshore tax sheltering activities of marginal or questionable legality, it would reflect adversely on his character, at least in my opinion. It is for this latter reason - character - that modern Presidential candidates have generally opened up their finances to public scrutiny.
As George Will and many others have written, the fact that Romney refuses to do so leads to the inevitable conclusion that he feels the blowback which would result from the public airing of whatever is in his tax returns is worse than the heat he is taking for not releasing them. It seems all but certain, for this reason, that the returns contain something very negative, or he would release them.
The thing is, you don't really want any of this explained in my view. You, like so many Romney apologists, prefer to treat this as a non-issue because it makes you deeply uncomfortable. It's really unseemly and intellectually dishonest.
As an aside, it's remarkable how you are so attached to using a word you don't know how to spell. It makes you look foolish IMO. "Loony" and looney" are permissible spellings - "loonie" is not (unless you're talking about the Canadian dollar, which as far as I know is not relevant to this discussion).