IS there a need for an air/water intercooler system? Probably not but you will have to determine your needs and performance goals.
Porsche designed the water/air inter-cooler for the early 934's out of necessity. Rules for that year did not allow for a large air to air cooler and or modifications to the deck lid to house a larger inter-cooler. Porsche reasoned a smaller air to water cooler could be used to achieve HP needs and satisfy rules. Air/air has been the main cooling of choice, 934.5-935's. With Porsche and racing it was and still is all about performance, reliability, and the rules.
I built a Laminova core based air to water inter-cooler system for my 934 clone.
My goal, fit a small inter-cooler under the early 930 decklid with the ability to still chase more HP. I have tried to make my system as efficient and reliable as possible and it does work well for me. But with that said I agree with what I have read on the forum that the negatives outweigh the positives. Positives really only being three things, smaller inter-cooler that can match cooling ability of a larger air to air cooler, possibly cooler charge air and ability to run a little higher boost because of it. Negatives are cost, complexity, weight, and increased probability for failure.
Air/air is the simplest, cheapest, most reliable way to go. Especially if you can achieve your HP goals with a small air/air cooler to fit under ducktail. For me it was more about the challenge than anything else. Hope this helps, Cheers Tad