Old thread, but still a question that comes up from time to time.
I bring an SPG or two in my save a dive kit. They stay dry. If, I were to find that my AI wasn't transmitting before a dive, I would swap it out for an SPG and deal with the transmitter issue later.
The need to carry both on every dive is just not there anymore, if it ever really was there. For some dives, it absolutely does make sense. For the vast majority of my dives, it doesn't. I dive mostly off of private (usually mine) boats, so if I had to swap just before a dive, it's not a big deal. Might be a different story on a charter where they are on a schedule.
But, AI transmitters today are very reliable, so in most cases a backup SPG is just not needed. I know that there are people that don't see the value of AI as they insist it's necessary to have redundancy. Yet, I never see divers with redundant SPGs. In many ways, I trust AI transmitters more than SPGs. Both can fail, but SPG failure mode can be scary. In some cases, the needle can just freeze at a previous pressure. That can mean that you think you have more gas than you do. When electronics fail, they usually do so in such a manner that there is no question that it's not working. If my transmitter were to fail, I would end the dive and head up. I usually have much more gas than the buddies I usually dive with, so would probably still do a safety stop, depending on my dive time at that point.
