It makes perfect sense to trade BTC vs Alt as BTC is the gateway coin that you need to go through to buy most alts. If you have $100 and but BTC with it, you could just hold the BTC, or you could trade alts. If you trade alts and make a 50% profit, that is 50% over the profit you would have made holding BTC.
If you measure success against USD and make 50%,but BTC goes up by 50% in the same time frame, you haven't really made any trading profit as holding BTC would have yielded the same profits.
As I tend to only trade with BTC (and alts bought with this) which was obtained via mining etc and not purchased at any particular price, I would much rather see trades compared to BTC and not USD.
That is exactly why I made this post. I don't know why people still wants to trade alts vs usd...
But what if BTC is unstable or falling like a stone?