I somewhat disagree with some of the recent statements. Here's my take.
1. The biggest concern on scrambles is injury.
2. A likely cause of potential injury is loose (moving) rock.
The more time one spends with rock climbing (real rock) the more likely one is able to see telltale features which help with safer choices (holds, route choice) on scrambles.
One also gains greater comfort and confidence, allowing these choices to be made in a more sober, relaxed fashion.
So while climbing class 3/4 will allow experience for same, climbing class 5 allows for more extreme experience, but at the same time, in a more controlled and safer environment (roped, more solid rock, one or two pitches, below treeline, etc.) This more extreme experience fast-forwards the experience process for class 3/4, without incurring the opportunity for 3/4 loose-rock hazards.
Steve alluded to this concept of balance between experience and exposure to risk:
1. Training highlights preventable mistakes.
2. Mentorship and group participation can teach skills.
3. GOOD books, i.e. Freedom of the Hills, can help.
4. Time in the field teaches valuable, applicable (but not perfect) lessons.
1. Climbing is dangerous...
2. Rocks move, feet slip, snow slides.
3. ...Time in the field increases the opportunity for #1 and #2 to catch up with you!
It's this concept that time spent gaining mountaineering experience helps, but time spent gaining mountaineering experience increases opportunity for hazard. Whenever you can gain the experience without incurring the risk, that's nearly "free" experience. In a roped, safe rock climbing environment with skilled partners, you can get this "free" experience. My advice is: take it.
PS... I don't want to get too much into the subject of risk level of rock climbing vs. mountaineering, except to say, rock climbing can be hazardous, though in competent hands, much less so than mountaineering, in my personal experience. After many, many years of rock climbing, I still know old guys climbing 5.11. After just four years of mountaineering, I've said goodbye to some wonderful friends, both young and old. We must realize we serve a powerful mistress. My advice: Do everything you can to mitigate risk. Learning rock climbing in a controlled environment with competent mentors may be one of the ways to mitigate a portion of the risk incurred in mountaineering.