I know what you mean and I agree that it is a potentially useful measure, but certainly not the best. I still think their own relative values all taken in fiat probably gives more useful information (about performance and value).
Don't forget, before this 2017 crypto bull run Maid had fallen way down from her 12c ATH of the previous bubble. To look at only the BTC change over the period would tell you it had been stagnant since we were at 20k sats at the end of 2015. That is of course not the case though, we dropped and rose by huge margins, and when BTC pumped we dumped, it didn't take us with it. MAID climbed back slowly off the back of it's own work, it wasn't dragged up by BTC values... although it was dragged up by the general crypto explosion
I'd say the only time to measure in BTC is if you're margin trading, at least then you are necessarily working with BTC as a baseline. Otherwise I'll make the argument for fiat all day long.
Each to their own though, I just see very little use for the BTC yardstick in my day-to-day in crypto since I stopped doing much trading.