Why not the piano as a reference for bass

The difficulty in using piano as a reference for bass response is that very few pieces stay down in the low bass for long enough. And the bass part is usually complementary to what's being played in the right hand so cognitively it's difficult to just listen to the bass part by itself. And if it' a grand piano, it usually has a lot of reverberation so the ADSR envelope of the notes is very different to an acoustic or electric bass. But of course, piano is still useful for gauging the bass response of system at frequencies that a string bass does not reach. Synthesisers are arguably even more useful. 

Okay so I didn't  clearly articulate my opinion of the extreme low bass on the piano..YES I know it is a stringed instrument but again I stand by my take on it's lack of texture compared to a plucked or bowed bass..
 I believe @toddalin reply explains exactly why I hear it as one note thump thump instead of the resonating,detailed and expressive bass I hear from say a cello or stand up double bass..

problem with the piano as a particular bass instrument is its relative lack of portability. that aside, unless it is miked closely, the bass fundamental lacks sufficient volume to poke through a dense mix. that said, the Beatles used the left hand piano notes as the bass line in at least one song ["for you blue"], and pianist Jess Stacy used his left hand as the bass line in several songs performed in concert with Benny Goodman at his seminal '38 Carnegie Hall jazz concert. unfortunately in that case the abysmal recording quality resulted in Gene Krupa's booming kick drum and floor tom drowning out most of Stacy's otherwise well-done bass line.

I still think of Ray Manzarek playing all Doors bass lines on his foot pedals.  Always correct bass, always Awesome.  Still play them occasionally.

Gimme the bass at about the 9 minute mark of Russia on Ice by Porcupine Tree, audiophile nerds.