It makes perfect sense to trade btc vs alts as btc is the gateway coin that you need to go through to buy most alts. If you have $100 BTC and you trade it to alts. If you make a x2 profit so you make profit of 100 $ BTC.
i think you trade to get udst profit with this way , I don't keep my trading capital in BTC, and I don't trade to earn more BTC. I trade to earn more USD.
So everyone have their own goal, but in those trades you constantly sell and buy bitcoins, for another alt or for dollars, that`s trading. More bitcoins is more dollars and vice versa.
I trade my bitcoins to buy altcoins short time and I have couple that i wait to grow, some holding. I will see this alts when price reach some top in my opinion and buy back if I think that price will bounce back.
There is to many factors to consider in trading, talking in general doesn`t have sense to inexperienced man in crypto world. For better advice please give more infromations about specific trades and alts you consider for your trade.