I removed my SPG and depth gauge back in 2003 or so when I get my first dive computer. At the time it was a ProPlus 2, so not wireless, but AI nonetheless. I traded that in for a wireless Oceanic VT 4.1 in 2013 with a PPS transmitter. Didn’t reconnect the SPG then. I’m now using a Perdix AI with the same transmitter.
When looking at my downloaded logs with the VT4.1, I noticed a few points on some dives where the DC would lose connection for a bit. I only noticed during the dive once. It cleared up right away. No issues at all with the Perdix and the same transmitter. I had been occasionally changing HP ports and wrists to minimize the lost comms. That seemed to help as my last dives with the Oceanic were solid.
As far as batteries go, I don’t tend to trust batteries all that much, but the transmitter batteries have been great. I change batteries each year, and carry a spare in my save a dive kit. Haven’t had to use it between dives though.
For the diving I do, I see no reason to have a backup SPG on the reg. If I’m on a dive and the transmitter dies, I’ll try to reconnect first, and if unsuccessful, I’ll end the dive. If diving in overhead environments, having an SPG as backup makes a lot of sense, as by their nature, a direct ascent is not possible.