I can agree with that to a small degree, in that you DO get used to an instrument's behavior over a range of market conditions. But to say that one instrument has only a few worthwhile setups per week is simply not true for all instruments. I mean, look at *insert any instrument symbol except class IV milk here*. Many setups each day on all of them in this highly-ranged market. But if you can expand your view to a few more than just the one instrument, you give yourself more options if one instrument lags in the number of setups over given period of time over the other one.
Again, if someone said all they trade is ES futures, you would not say shit. Many do that SPY is the SAME THING do you not get that? , or are you deliberately dense