That's my point.
Not everything has to have a purpose that you define as a good one. Titillation alone serves a purpose.
And my point, at the beginning, was that the artwork should serve a purpose. And to give the artist some consideration that certain purposes may not serve the game product's interests well. However, the OP has since posted what the product will be and the style of artwork, the nudity in the case of that artwork was an item I agreed would serve the work well.
So, overall, we're arguing over the same point
It can be expensive to print, but most RPGs that are put out these days are sold as PDFs. Printing costs don't really have anything to do with whether the art contains nudity or not, though.
Believe it or not, but printing costs remain very much a real concern even with PDFs. In the case of PDFs, the "printing" cost is the cost of hosting it and dealing with downloads, and in that case you tend to have practical limitations on how big you can make the PDF before people start refusing to download it. That's why PDF-oriented companies tend to produce books that are much smaller in size or containing much less artwork than print-oriented companies.
So what you really meant to say was that art must serve a purpose to justify the cost in a business context? I won't argue with that, at least when it comes to D&D and Pathfinder, but that's a bit different than saying that it must serve a purpose in general.
As I said before, most RPGs that are put out today are distributed as PDFs. Most will make very little money - certainly not enough to actually cover the effort and time put into making them. Though some people have unrealistic aspirations of making a living off games they create, most are more interested in making something cool and having other people play it. That's why there are so many free RPGs, as well.
It probably doesn't make financial sense for WotC or Paizo to include much nudity in their books, but those two companies bear little similarity to most RPG-producing companies and individuals. They're big and they make money and are the games that are most extensively played, so they have to appeal to the widest possible audience, which is (sadly) generally very nudity-phobic. That's only two games, though. There is a whole world of RPGs out there.
You're going to find that most RPG companies shy away from nudity. Numenera, which includes what is probably the best-written and most mature book that deals directly with sex that I have ever seen for an RPG supplement, doesn't include a single bit of nudity within any of its books. Including the sex book. Shadowrun has prostitution very much part of its setting, yet also avoids nudity like the plague. Savage Worlds, which frankly very obviously sells itself at least in part on titillation, tends to have
less showing of skin than Pathfinder. I think you'll find the most nudity in nWoD, and even then it's maybe two pieces hidden deep in random supplements; the rest of the time, what little artwork there is tends more towards covering. And DnD has gone so far away from it that what little they have actually stands out more as a result.
So, if anything, on the nudity front, Pathfinder is probably the most risque of the major RPGs.
For the minor RPGs, I find most of the ones I've seen tend to either be around the level of Shadowrun or Pathfinder, though quite a few also tend towards DnD and Numenera. It's been a long time since the Book of Erotic Fantasy came out, and these days it seems products like that simply don't seem to be around as much anymore. So, for anyone producing a supplement, they have to deal with a market that is a lot more conservative on the nudity issue than it once was. We've come a long way from when nudity could be found in DnD.
And, the above is not to say that nudity shouldn't be presented; it's to say someone producing a product today has to consider whether or not it would make any money with how the market currently is.