But social skills involve learning to get on with equals as well. Equals will not be so "kind". They might not be nasty say as adults, but they will simply avoid that person.
In real life, you don't "just" associate with people the exact same age as you. Most people associate, even in their own social group, with people that can be a few years younger or older than them as well. My dcs socialise with children that are the same age, older, younger, as well as adults. The older children have fairly well developed social skills and are tolerant of the younger children, so clearly this model works for many children. I think it's important for children to be able to socialise with varying ages and see that they have equal value and varying important things to contribute. I also want them to understand that they as individuals can have some say in how they socialise depending on how comfortable they are with each particular situation. I encourage socialisation, but I also allow them to step back from it here and there when they are not comfortable. That's also an important developmental step, IMO.
As an adult, I have friends of many different ages. These people are friends based on shared interests, values, and life experiences. Some are my age, but many are not. I don't actually check if they were in my school year before I consider a friendship. 
Some people retain friends from their school years, but some people do not.
Children can't realistically go through life only learning about things which interest them. At some point they have to learn boring but useful information!
True, but only to a point. Some information learned in school is not useful, nor is it necessary. Other information that is necessary isn't actually learned in school. My two that home educate are autistic, and school was simply not the appropriate setting for them. We have a semi-structured approach that works well for us. Others that I know do the unschooling, and it seems to work very well for them.
I think it's important to remember that all children are different. Just because you don't necessarily "see" education taking place, that doesn't mean that it isn't. We often get some of the more structured stuff done early in the morning or in the late afternoon, so that we can go out during the day for activities, socialisation, whatever. Some days we have more of a "low key" day and stay home (like today), especially if we have just had (or plan to have) a few busy days in a row. My children need down time to cope. Attending school all day 5 days a week did not allow this. Like I said, different children, different needs.