Not sure I agree with this.
I installed (myself, very easy) the Xantrex Link 20 last Fall which monitors my 690ah house bank on one link and the cranking batt on the other. This gives me a quick glance at the voltage health of both batts; among other key data points. Case in point, after entertaining 6 people for a daysail on SF Bay, I went to start the engine, only to realize my cranking batt was dead. In quicktime fashion (and without my guest knowing) I switched the cables to my house bank and cranked over the engine. Had I had the monitor installed at the time, I would have realized my cranking batt was bad before leaving the dock. Now I constantly check the health while underway. I think this is a major safety tool, as having your engine at the ready in an emergency is obviously important.
Regarding the shunt. Mine came with a 500amp shunt that contains 4 posts. I'm not sure why there are four. One post is used for the DC system negative, and two others are used for the negative cables coming from each battery bank. The Link 20 monitors voltage, amps, AH and provides a time estimate for both banks.
I would definitely give an endorsement for the Link 20. You might want to compare both models, as they are priced very similar.