Hi lucy
I've been doing 50/50 with my kids for years. I work full-time, so the extra time I have is basically a few evenings a week, one or two weekend nights and a day or half-day on the weekend. With that time I see friends, do housework, spend time with my OH, read, shop, do my hobbies and I have done some courses. TBH, it's not that different to what I'd be doing if my ex and I were still together apart.
I'm asking because I was advised against 50:50 as not being in the best interests of my child. The official line is that it's no longer what's "fair" to the parents, but what's best for the child.
It's always supposed to be about what is in the best interests of the children and the received wisdom in the UK courts is to stick to the traditional and old arrangement of children living in one house with visiting every alternate weekend. I find it really alarming that the "normal" set-up is not subject to the same skepticism and challenge and rigour in defending itself that shared parenting. I am sure that for many families the traditional model does work, but it is far from clear that it's universally better for all children in all families. The courts and social services etc, who hold the view that it's "not a good idea" only deal with a small minority of child contact arrangements, the ones that can't be worked out amicably by the parents, so it's unsurprising that they hold a cautious and conservative view.
Would you want to be changing houses every few days?
If I was a child and the alternative was having to be separated from one half of my whole world for two weeks, then yes. That's certainly my children's view. This horror that adults have about the idea doesn't necessarily reflect the way children experience it. But it does require that we are well-organised and that we ensure they experience as little disruption and as much flexibility and understanding as possible.
To my kids, they're not "changing houses", they're just going home.
Whenever the topic of shared parenting comes up, people raise their arms in horror and inevitably someone will talk about living out of a suitcase or being shuttled "back and forth" and I despair because the none of that stuff happens (in our family) and it just seems the whole child contact issue is so polarised with people in these two very divided camps with no room for discussing common ground or learning from the other 
teapot The two sets of friends argument made by your ex's barrister seems like a strange thing to argue in favour of 50/50, however I do think it's quite usual for all children to develop different groups of friends especially as they get older. My kids have friends from school, friends from their activities, friends they know from when they stay at grandparents, friends they were at nursery with and so on. My ex and I live close enough to each other that it makes no difference to which friends of theirs they see and when.