So here's what I think.
Children develop at different rates right? For instance, sometimes a 9 month old begins running while a sturdy 18 month year old has yet to take a step. Eventually though, almost all children who do not have other disabilities learn to walk and run.
I feel the same way about yoga class. It is the rare beginner who can take in all that we are attempting in a yoga class (new breathing techniques, developing still and meditative mind, complex asana/pose directions) and also cope with noise and distraction. For this reason I attempt to keep my beginner classes as soothing and mindful as I can. I want people to have the best possible experience that they can and a soothing environment is key. Some people can handle extraneous sounds (like cell phones) but most are distracted by it in the beginning.
(Having said that though, I never make people feel badly about their phones ringing accidentally in class because that is contrary to the precepts of loving-kindness and acceptance that I personally aspire to in my life and practice.)
For the more experienced practitioners however, I like to add a little more zing to class. Do you remember the first time you drove a car with the radio on? Did it take a little getting used to? Probably. Did it eventually make the drive a little easier or interesting? Maybe. Changing the environment doesn't necessarily "negate" from the overall goal which is to be a good/focused driver no matter what you hear on the radio. Do I think that driving with a cell phone clamped to your ear is a good idea? NOPE. Moderation in all things, even distractions.
So in my intermediate classes and even towards the end of term in my beginners classes I encourage a little silliness or playfulness. Going off focus and then regrouping is a fairly important skill to cultivate both on and OFF of the yoga mat. Do I think it should be a free for all? No. Do I think there is a place for laughter and even a little zaniness in a yoga class? You betcha. Meowing and mooing through Cat/Cow sequences is liberating and adds a whole new layer of focus to the pose through new sensations and feelings. I actually get requests regularly for this one because it is such fun and feels so good. By the time we get to relaxation (corpse) pose at the end of each class we are all regrouped into that sacred space of peace and release. It is from here, irregardless of what else has transpired in class, that we move forward into our day with equanimity and balance.
Just my two cents worth of course. Perhaps too much mooing and meowing has addled my brain...