I can't resist chiming in when Nick starts talking avionics.
But this has actaully been a hot issue on a number of airframes, driven by a lot of tradition (this is the way we've always done this.) The back up CDI has been a vestige for some time. In most modern electronic (EFIS) cockpits it is no longer required - as can be shown by ARP-4761 hazard analysis. (Loss of all "radio" nav and com in IFR typically being "hazardous" while loss of all radio Nav with com remaining being "major") Most modern EFIS designs could support "Hazardous" level for loss of radio nav without a backup CDI. For example, on the S-76 you can recover from a failed symbol generator by selecting reversion (as Nick mentioned), or from a failed display tube by selecting composite. And that's all still all at the pilot's station. The combined probability of both failures is easily better than 10e-6 and likely approaching 10e-7.
It was a different story with an electromechanical HSI with a failure rate of less than 1000 hours. With no ability to recover information on another display at the pilots station, the back-up CDI was essential. However, even with electronic displays and all their reversion and redundnancy, old traditions die hard with our regulatory agencies. For Example, the EFIS-equiped B-430 was initially required to carry two backup CDIs - one for each pilot (so much for single pilot being the driver in that case.) That has since been reduced to one after significant effort.
So why do we still provide a back-up CDI on the standby for electronic cockpits? In some cases - just because it's there. That was pretty much the case for the BA-609 architecture. We didn't need to by hazard or worload analysis, but since the GH-3000 had the capability, and our pilots agreed it didn't add too much clutter, why not. After all, as an engineer, you never walk away from the oportunity to provide extra cabapility when it's free.
Why the connection to NAV-1? I've heard the arguments that this allows the pilot to cross-check NAV1 while he flys NAV2 (with pilot on the right) - but unless you are flying CAT II approaches, this is a rationalization and not really an issue. The real reason is that most aircraft power distributions are designed to protect NAV1 & COM1 because these were traditionally considered the pilot's radios - something from the starch-wing boys. In the 412 for example COM1 is on the emergency bus, NAV1 is on essential, and COM2/NAV2 are on non-essential. And in the 412, you don't lose the essentials unless you select EMERG LOAD (Which you'd only do to get extended 90-minute battery only ops.)
I and several others have attempted schemes to protect the pilot's NAV/COM for helicopters, but with no success. The problem is that we tend to sell VFR versions with only one NAV & COM installed. And it's just too wierd to install the number 2 system first. It confuses the hell out of pilots, and it makes the guys who do Circuit Breaker panels go nuts. So until the day when we call it Pilot/Copilot or Left/Right rather than 1/2, NAV1 will remain the limp-home, last ditch, protected pilot's nav radio.
Sorry, that was probably a bigger brain dump than you were expecting.