For safety, all sorts of climbers consent various standardized forms of climbing. Standards for equipment is probably the most objectified standard. Whether you climb rock or ice, you need to use ropes, helmets, harness and so on in each of their appropriate sizes and dimensions -- This is arguably one basic ethics in their community.
If you climb rock, you can use a rope in a diameter of 9mm - 11mm nowadays. In an alpine terrain -- that may take more than one day on ice and rocks -- the choice of rope is significant, due to an ambivalent choice involving both the weight of the rope that should take much labor and the potential cut off of it due to rock or ice fall or climber's falling. Put simply, if the rope is too thin, it may be easily cut off, whereas, if it is quite thick, it is too heavy to carry on with all other equipment in their back.
Some mountaineers (particularly who climb in the Alps) prefer to use double-roped or twin-roped system. It is good for avoiding cutting off the rope (for using two at once) and for rappelling down. Although in the system the diameter of the rope can be reduced to 7.5 mm and the weight of one rope would be around 2.5kg, the paired ropes in total weigh still up to 5kg, which is heavier than 4kg in 9mm-single-roped system.
Then, I ask: what is safety? when can a climber be safe in climbing?
It is believed that climbing is dangerous. But, it seems to me there is NO universal agreement what is danger. Kazakhstan mountaineer Denis Urubko says: "Climbing may be dangerous, but life itself is dangerous. What different between the two is, the former only make it observable, while the latter it is hidden."
It may be hasty if you say "climbing without rope is too dangerous," for there are not a few people who climb without rope and who feel completely comfortable. If they felt it was too dangerous, they won't do that. Likewise, although using rope thicker than 9mm is now an almost universally-accepted standard for safety in mountaineering, climbing with a rope in 7mm is not necessarily more dangerous.
7mm-diameter rope is therefore an ideal. It is just one step over to those objectified standards, community agreements, habitual assumptions. Yet, it does not have gone too far from any consentable boundary of their community for safety. It is not too bizarre as handful ropeless climbers almost always deemed. Instead, it is a realization of the passionate progression over one's capacity -- the maxim of climbing. Most important, it forces the climbers tied into it to ask the standard of safety, of danger-life relationship, and of the meaning of climbing, while their flowing into series of climbing activities.