"Necessary"? You could dive with no computer at all.
Haha... true. Save yourself at least a couple hundred bucks and dive tables.
Some computers with alarms and lock-outs and ultra-conservative algorithms are probably better suited for the 1-5 dive a year vacation diver that more than likely forgets just enough to get hurt or dead. The Petrel will give an inexperienced diver enough rope to hang himself.
AI (in theory, since I've never thrown down the cash to get it) is nice as a back-up in case your brass & glass SPG fails. But the AI is probably more likely to fail first (just maybe not at the same time).
Digital compass... again, a nice back-up if you either lose, drop, break, or forget your "real" compass. In that case it's nice to know it's there as long as you didn't forget your computer.
I would have figured for the minimalist crowd a digital compass would mean one less thing you have dangling from your BC. That's the reason I would like to have one.
I'm pretty sure most if not all "recreational" computers have something to keep you from getting bent if you go into deco. It helps to know when it's telling you this and what to do when it is.
I think computers like the Zoop are more complicated to use than a Petrel. I have a D4i and I think that's even easier to figure out than the Zoop. I think the Zoop also automatically defaults back to air after a dive or after it's been shut off. I find that pretty irritating, but I guess someone somewhere likes it.
Given the release of the Petrel 2, you might be able to score an
old Petrel for a good price... if you can appreciate what you're getting for the price and at some point realize you may have jumped the gun on the whole "I'll never do...".
While I don't have any personal experience with it, my opinion is the next option down from a Petrel for what you plan on doing is the xDeep Black w/ the EANx upgrade. That version is about $200-ish less than a Petrel but gives you what you appear to be asking for.
AI is not necessary. An analog compass works just fine. So why a Petrel?
Support from Shearwater is second to none.
Did I say the way they support their product and provide service is one of the best of any company in the industry and out? If not well, they do.
Agreed on everything but "Oh by the way, if you're thinking about buying a Petrel, wait another month and you'll also get a digital compass". Or at least tease to "we're releasing a new computer with a couple new features"... like an iPhone. Nobody knows for sure but you know there will be improvements.
I've heard they didn't even share that little tidbit with some of the dealers before ordering. Take that for what it's worth. Maybe that's not an issue with service/support as much as marketing.
In hindsight, I should have seen it coming because the NERD came out and DEMA was right around the corner so they had to hold something back for that. The one feature that stood out in the NERD was the compass... the logical step would have been that it was going to be seen in a wrist top. Again, hindsight it adds up.
I really like my Petrel. I have limited experience with their service/support because the product works the way it's supposed. I had a brief issue with connecting to Shearwater Desktop which basically was resolved by reinstalling it.