Not really irrelevant. Consider this example: let us say you have coin X whose value is .5 in Bitcoin. And assuming that the value of Bitcoin is 100 USD per 1 Bitcoin. If you have 1 coin X, that means you have 50 USD because that is half a BTC. Now, this is to show you that the value of Bitcoin is very relevant. If Bitcoin's price goes up to, say, 200 USD and your coin X is also growing in BTC value, reaching .75 in Bitcoin's value, then all in all your worth has reached 150 USD. Had your coin grown to .75 in BTC but BTC remained at 100 USD only then your gain would not have reached that high. I hope you get my explanation.