The worst things about Trump are not big secrets. I think it is dumb to make any deal at all of this urine story, because nobody knows if it's true, probably we’ll never know, it's not worth resources to try to find out if it is true, and it elevates the unverifiable story above the historic conflicts of interest that are being exposed before our eyes, horrors with zero deniability. We don't know if the pee stuff is true, but the nominations of Tillerson, Sessions, Tom Price, Andrew Puzder, Steve Mnuchin, etc. — that is all real and undenied. Trump's directly profiting from the presidency is for real. If you make a big deal out of the urine stuff it's on the level of talking about the Clintons killing Vince Foster even after Kenneth Starr said it was a suicide. It's not quite on the level of speculation about Obama's birth, since that is intentionally ignoring the truth, but it is making a big deal out of something which is total speculation at this point. And that makes attempts to educate the public about the important issues harder.
In political struggles it's a good thing if you can be distinguishable from your opponents. When they do something wrong (spreading unverifiable stories) it's good if you have a clean record, and not just be better than they are, but clearly better than they are. It's true that Trump is gross so something like this “scandal” is arguably more plausible, but even though I can't stand Trump I don't particularly think the pee story is true. Somebody who is favorably disposed to Trump is far less likely to believe it.
Why not stick to issues that are relevant, proven, and so much more important? I think every time anti-Trump people bring this up in a political setting (as opposed to telling jokes with friends) it helps Trump, it doesn't hurt him. If it were established fact, then obviously there'd be no end of joking and tweeting about it until the end of time, but since it is not, and almost certainly never will be, it's a waste of time and energy.