It is unrealistic to expect people making frequent trades to carefully document every trade (unless perhaps that is their profession). It's even worse for cryptocurrencies, since the value of the outcome of a trade is entirely relative to some other more established asset (such as US Dollars).
If you trade USD for BTC, then some time later trade BTC for XRP, then liquidate that for something more obscure (like cV), determining the value is really difficult. You would have to lookup the currency pair rates for each pair in a given trade, as well as some chain of currencies which can be associated directly with USD. Not only would that be highly inaccurate, but it could easily be argued very differently depending on which exchange(s) you used (since at any given moment, the difference of rates on different exchanges can be very significant).
I think as far as the IRS is concerned, unless you're obviously getting rich, if you can just show you made reasonable effort to assess and report what you earned, they'll be satisfied. But if they think you're trying to fool them, they make take personal offense and put far more resources into nailing you than you think it's worth in lost tax revenue.