Not really. You just get a prorated refund on the AC portion you didn't use and can apply that towards the purchase of a new AC policy.
According to the person I spoke with on the phone when I bought AC+ a couple days ago, there are basically 3 options if you get a different phone within the 2 year AC+ period :
1) Transfer the remaining AC+ coverage to the new phone (presuming you get another iPhone)
2) Get a pro rated refund of the remaining amount (whether you get an iPhone or not)
3) Let the AC+ stay with the phone you bought it on (eg if you are giving to a family member or want to use it to try to sell for a higher price).
He said that 1 and 2 require you to call to make the request, otherwise 3 is the default.
Regardless whether you are using 1 or 2, you're basically talking $50/year plus $79 per incident(up to 2 replacements)
So for those talking about $179 vs $109 for a screen replacement, it is more like $129 likely for a brand new phone ($50 AC premium + $79 incident fee) vs $109 for an Apple-provided screen replacement (and possibly having to deal with other damage). So really you're talking about roughly $20 more for a new phone (and probably a lower overall cost under AC+ if you ruin it with a "water incident").
At the end of the day, it is insurance and Apple is going to make money on it. But you have to look at it from your point of view (whether you think it is worth it from a cost benefit standpoint, peace of mind standpoint, etc) and not worry about Apple's point of view (because as a whole they are making money on the deal).
If you think about it, Apple's cost on the phone (which I have no idea what it actually is) may not be much more than the AC+ premium, so even if they have to provide replacements to people, they may still come out ahead or at least be close to break even. But again, you're not Apple so you have to do what you think is in your best interest, which will be different for everyone.