Keeping the foreskin rolled back is widely practiced. I know this: I run a web-based interest group. Some guys do it as the mood takes them, others (like me) keep it back 24/7. A few points:
1. The issue of loss of senstitivity is debatable: it's been a recent thread on my group, and we all agree that it's a myth. Once the penis is erect and the glans 'inflated', it's as sensitive as anyone else's. It is possible to cease retraction at any time, and leave the foreskin in its 'natural' position, and the moistness and so on return fairly quickly.
2. Likewise the 'design' argument. Loads of blokes have foreskins that naturally cover 50% or less of their glans. Asian men especially are noted for having short foreskins which retract completely, naturally exposing the whole glans.
3. As for 'why would you do it?'', well, why do any of us do anything that gives us pleasure? 'Just because I get a kick out of it' is the honest answer (and one all too rarely seen on here).
4. I do know a couple of guys who have such large helmets even when soft that it is more comfortable for them to keep their foreskins pulled back. I also know several medics who are 'retractors', and who suggest the practice to patients from time to time.
5. The other 'scare' that is often raised is paraphimosis (getting the foreskin stuck behind the glans). If you foreskin will roll back naturally, then the likelihood of paraphimosis is very low.