Well, AI isn't "necessary", because you can use a simple gauge. Many experienced divers don't like consoles, because they prefer to have their dive information on their wrists, so you're looking at a transmitter setup to get AI. Transmitters are expensive (although a lot of advanced divers dive VERY expensive computers, anyway). Transmitter-based AI setups can be awfully finicky about syncing, and if it doesn't sync, you don't have AI.
I dove with a simple SPG and a wrist computer for a long time. Recently, my husband has "corrupted" me, and put a transmitter on my single tank setup. I've had it about six weeks or so, and I'm still quite ambivalent. When it works, it's convenient. But it requires a pretty specific sequence of startup to ensure that it syncs, and I frequently forget to do things in just the right order, so I end up in the water without the information, anyway. Still not sure it's worth what it costs. Ask me again in a year!