The impending release of the 12.5-13mm CMC/Harken Clutch is occupying a lot of social media space right now. It's also bringing out some derision against those that are using those rope diameters. "Who still uses that?" "Why"... etc.
The simple answer is - A LOT of teams and for justifiable reasons. When I ask around to people selling rope it is clear based on their sales that easily half of the US Fire and Rescue market is on that train and it's fine.
Why haven't they switched to smaller ropes?
1) Saving weight doesn't matter to them. Unless you are carrying the rope significant distances the cost/benefit doesn't lean towards making the change.
2) Change is expensive. If you are managing a large agency the cost to make a wholesale change is daunting and in some situations financially irresponsible. How much rope and other diameter specific hardware has to be cycled out of service? How much training needs to be added to meet the change? Is that time/money better spent somewhere else.
3) 12.5-13mm isn't unsafe and a 13mm has about 20% more cross sectional area than 11mm. Given the same rope construction that can equate to greater cut and abrasion tolerance before the strength is critically compromised. (Yes, you should protect your ropes from hazards at the get-go, and yes, rope construction continues to improve.) The "old" way may in some situations give greater safety margins so if the weight isn't a downside to that operational reality why give it up?
12.5 rope isn't going away anytime soon and it doesn't need to. With continued evolution like the Petzl Maestro and CMC/Harken Clutch supporting it we will be seeing it for years to come. However nearly every major rope manufacturer either has a G rated 11mm on the market or one coming very soon. As equipment continues to improve the shift will happen naturally but remember the costs and benefits of change are unique to different situations, motivations, and budgets.
Now, if the situation calls for going light then it get's interesting. Lightweight certainly has a place and we'd love to see these same well engineered technologies move into the skinny rope realm. Please. Going fast and light has a time and place and descent device compatibility is the greatest challenge. With increased knowledge, smarter rigging, load reduction, and force limitation systems in play lightweight rigging can be safe and effective.