These PDK vs manual debates are always amusing.
Porsche will use whichever transmission suits them best and overlay a marketing spin on that choice should they need to. When they offer manual only such as in the GT4 they may say it's for enjoyment and a true driver's car should be manual. When they offer PDK only, such as in the 991 GT3 they said that a manual just would not allow them to advance the car to where they wanted to take it. That wasn't necessarily referring to lap times but all the components they wanted to mate together to deliver a particular height of driving experience.
We'll see both manual and PDK for as long as:
Porsche see a market for it
Or mechanically on some limited models it's easier to do without, without affecting sales (might there have been an issue with packaging the 3.8 and PDK into the 981 (I don't mean space) that it just wasn't worth overcoming for the numbers.
Or would it make the GT4 climb far too high in taking scalps in the 991 catalogue.
Personally I think the latter. Porsche likes to carefully engineer the performance of its cars in line with the price tags, e.g engineering a tenth of a second between Boxster S and Cayman S with tens of HP difference (which must take more effort than just dropping the same engine tune into both cars (and giving them the same price tag)). It wouldn't surprise me if they omitted PDK from the GT4 because it would be a few tenths the wrong side of some benchmark vs other models. That is not to say faster is better or whatever, just the way Porsche consistently do it. But the marketing brochure will give a marketing reason why it is manual only.
Mazda are in the business of selling as many cars as possible, just like everyone else. They'll have their reasons for the MX 5 being manual only. Do they have a DCT transmission in their locker or are their auto boxes of the old type which are seconds slower and worse on MPG and emissions. Taking one of their other cars at random, the latter appears to be the case. I wouldn't expect them to put that type of box in their MX 5 either. Haven't checked their whole range but have just read that Mazda opted not to go with DCT some years back and are persevering with the traditional auto. When they have one that they feel will sell more MX 5's maybe they'll put it in. Or maybe they're putting the car in a niche slot because everyone else is moving away from manual. I don't know. But I don't think the fact that the MX 5 and Polo are manual gives us any steer as to why Porsche makes the GT4 only.
The point being, you can interpret any choice any way you like. If Porsche put a turbo diesel in the Cayman as Audi do in the TT, there'll be some people saying why that is the best choice Cayman, for whatever advantages it has over Petrol and negate all the petrol performance advantages. The fact would be Porsche would have only done it to sell more cars, even if the brochure says something different.
We'll probably have a petrol vs diesel debate now. Sorry :-(
