The primary reason you would want to add a mechanical switch is so you can close up your AV bay ahead of your launch, although I personally open up my AV bays and check everything before every launch. This is an especially good idea if you use screw terminals... I have seen them come loose.
In terms of safety, the Eggtimer WiFi altimeters are some of the safest ones you can get... BOTH sides of the ematches are essentially "dead" until you remotely arm the rocket on the ground, the altimeter detects a launch (usually at 200' AGL), AND the first channel output event occurs. There is no "battery common" connection to the ematches, instead there is an electronic switch on the deployment power that doesn't turn on until the first output event.
And as Mr. Schwantz alluded to, this issue was resolved by TRA in 2020... electronic switches (whether wireless, magnetic, or whatever) are now considered equivalent to mechanical switches. They DO use a little bit of current, of course, which is why you don't want to connect them to the battery until you're ready to use them. If you forget about them they will eventually drain your battery.