Picking 15 hcp 'out of the hat' was actually pretty fair as 14-15 is supposed to be the average hcp for a UK male.
But 15 hcp'ers are 15 hcp'ers for a reason, or rather several reasons, including but (as lawyers say) 'not limited to' - lack of talent, no golf brain/no course management skills, over-ambitiousness, awful short game/putting.
One category of 15 or so hcpper that Sean has very correctly identified, and that Tom also mentions however, is the savvy player. Probably someone who previously used to be a lot better player but due to age/strength now doesn't hit the ball far enough to still play to a lower hcp, and being older and therefore probably retired with lots of time available, plays a hell of a lot golf so has a very 'sharp' game. These guys are dangerous opponents and really know how to 'use their hcps'. Very much down the middle, down the middle, pitch/chip, 1 or 2 putt and an easy bogey/net par goes on the card over and over again.
What makes them play/score worse though? Narrower fairways and more hazards etc? No, their straight and savvy so not much effect on their game/score. I would suggest what would effect them would be a combination of things such as longer 'no-roll' fairway grass, raised 'can't hit it high enough and stop it quick"' greens, forced carries and long par-3's (no SI shot). Mostly exactly the things which came up on the 'What design feature do you dislike the most' thread. The saving grace for other 15 hcppers however, is that these guys tend to focus on Senior's golf. Phew!
As to the 'defend par' element, par and medal play have a interesting effect on golfers brains as in the "if I don't make par I'll drop a shot" mentality. Most amateur golf is 18-holes and someone is most likely always going to have a 'hot day' and there's only ever one winner so without playing aggressively a player is pretty unlikely to win ('win' as distinct from 'place' if you see the difference).
Now over 36 or 54 or 72-holes however, the game plan changes completely. Screw-ups hurt if you're near the cut line, and you ain't got a chance of winning unless you make the cut, so to be in any way remotely successful you better play brain golf and use that 15th club', the one between the ears. Perhaps some of the tour or ex-tour players who post herein would care to comment on this aspect.
atb