This is coming from the "beware the man who only owns one gun - he's probably pretty good with it".
And it's total BS. You can have one bass and suck. You can have many basses and be great. And vice versa.
It comes down to the player, not the instrument, and most definitely not the number of instruments.
Let's take it a step further - what about multi-instrumentalists? There are some great ones. Some people have a combination of talent and drive that allows them to do more. But by your friend's logic, they'd really really stink. Geez, not only different string spacing and ergonomics, but different strings! Different number of strings! Different (OMG) TUNINGS! And heck, maybe they play piano too, or (lord forbid) a brass or wind instrument!!
David Lindley won the Topanga Canyon Fiddle Contest, starting when he was a teenager, for so many years running that they made him a judge. All the while playing guitar (lots of different ones), lap steel, banjo, and just about anything else with strings that he could get his hands on.
Didn't seem to hold him back any. We could point up other examples as well.
Oh right, Marcus Miller plays bass clarinet pretty well. Poor guy. Just think how good a bass player he could be if he had focused and gotten to be "best friends" with one instrument?
I'm starting to feel bad for Marcus now - all the work he missed...
