Transmitters have been around for a quarter century now. They aren't a new fad. They're steadily replacing mechanical SPGs the same way dive computers have largely replaced plastic dive tables. Transmitters are more dependable, more accurate, more durable, and more streamlined for sidemount than brass & glass SPGs. Although SPGs still have a couple advantages.
While first generation transmitters often had connection issues, that's no longer the case. With the exception of a negligent diver failing to monitory battery life, it's rare for a modern transmitter to lose connection for more than a few seconds. Even if that rare case occurs and the transmitter should fail, the display turns to garbage and provides an obvious and immediate indication to turn the dive within the Rule of Thirds. In contrast, when mechanical gauges fail they tend to stick indicating more gas than is actually available, presenting a potentially dangerous situation by overestimating remaining gas.
The transducer in a transmitter is accurate to within 2 bar/30 psi compared to the standard bourdon tube SPGs which are only accurate to within 15 bar/220 psi. That lack of SPG accuracy is a secondary reason recreational divers were taught to end a dive at 50 bar or 500 psi. The thick plastic shell and static internal parts of a transmitter are much more difficult to damage from impact stress than the glass face and moving mechanical parts of a SPG. For sidemount, the transmitter is tucked out of the way along the lower valve and tank neck and protected within the diameter of the tank rather than alongside it or above it. It is further protected by its position against the diver's body. For many divers, a transmitter is also easier to read on a backlit digital wrist-mounted computer than a SPG's analog glow-in-the-dark face on a short hose.
The benefits of an SPG over a transmitter are no need for a battery (although the hose & spool must be maintained) and a significantly lower price point. A high quality SPG costs just 20% of the price of a transmitter (US$70 vs. US$350 not factoring in customs duties or taxes). SPGs can also be more comforting for people less at ease with technological change. It's this final point which causes some divers to use a SPG as "redundancy" to a transmitter where they would never place two SPGs on a sidemount tank.