How many are using the factory amp meter

Looks like Del beat me to it. Here's my answer anyway.

The main reason for bypassing the ammeter is because it can be a weak link. ALL the current in the system runs through the ammeter. Yes, that's it's job but it is an older design based on '50s generator technology.

Let's say you have a wiring problem causing high resistance or a short somewhere. The potential for melting it goes up considerably since there is now unregulated current running through those puny gauge terminals. It's probably OK if you're on top of all the wiring in the car and know everything is in good shape though. If you re-did your dash, presumably your wiring is good.

The prime spot for problems is the bulkhad connector. Things come loose over time or get yanked apart by wayward hands, dirt and moisture gets in there, etc. Take the time to ensure all the connections through it are good and are packed with dielectric grease.

I bypassed my ammeter and am using a volt meter under the dash. I rewired my entire car though and did the bypass during that job. Just seemed like good insurance. I have little in the way of accessories though which by default decreases potential wiring problems.