I don't think there is a right answer to which is best, AI or SPG or both. There simply is the question of what you as a diver considers an acceptable risk. Even though I do not feel comfortable doing solo dives, perhaps because it was drilled into me from day one that solo is death (OK I am exaggerating for dramatic effect here), I don't see anything wrong with solo divers going out there and doing their solo dives and having fun doing it. Would I do it, no because I don't feel comfortable diving solo and I find it hugely rewarding to share my diving experience with my wife or someone else with whom I can debrief afterwards to find out what I missed and he or she saw. That's just my preference. Is there more risk in diving solo, sure. Is the Scuba Polizei going to lock you up for doing it? You never know, but I sincerely doubt it.
My point is, as a diver you take on a measure of risk every single time you dive. That measure of risk is a calculated decision about what you consider acceptable risk. If you feel that the risk of not having an SPG and having a transmitter or computer fail is acceptable given your dive plan parameters, your experience level, and historically known gas use stats, then by all means go for it. If you feel that having that extra safety backup, even if it just helps you to stay calm because you have something to look at when your electronics fail (for whatever reason), then having the SPG has value in my opinion. I do not think this choice is something that anyone can dictate to another diver as being superior or inferior; it's a personal decision about acceptable risk.
The OP mentioned that the point of considering ditching the SPG is because of streamlining. Some posters have hinted at this as well but I find it illogical to ditch your SPG in favor of an AI computer because it reduces drag underwater. Ditch your SPG if you want to geek out without your SPG out, no worries; stick to your SPG because you don't trust them darn 'puters, fine with me also; carry 'em both coz the air pirates will steal your air without you knowing about it, I'm fine with that too. But, ditch your SPG because you'll be slicker than a greased pig underwater and you'll have better air use because of it, errrr no. Just route the SPG carefully and appropriately, shorten the hose if you must, and I would argue that the difference between not having your SPG and just managing the way it is secured makes very little difference in the amount of drag that is reduced and that consequently allows you to improve your air use by any appreciable, consistent amount. You'd be better off getting into better shape at the gym and increasing your cardiovascular system's ability to function better when you dive IMHO.
