An indictment is not a successful prosecution.
And it does cost money...money, in my opinion is well spent, because in brings in skeptical people such as yourself with the power to check an overzealous prosecutor with simply arresting people on flimsy charges.
There's no defense because no one has been charged; and if you decided that the prosecutor is full of shit, then it means an innocent person is not disrupted or tainted with legal action. That they withold evidence is to protect their sources....which is a trade-off for them, because to withhold is to weaken their case.
I see nothing ridiculous about it at all, really. You're serving a hugely important role as a check on the ability for the executive to go after people. It's a rare occasion where the government is directly answerable to YOU. Hold them to a high standard. And if you think they're trying to pull one over on you, the people, send them packing.