I have made soaps full of shea butter (I don't know the exact percentage, since it was in a mixed product). The amount of shea butter in my finished soap must have been way over 10%. I will guess around 30% or something. I can't say that it does not lather, since my soap have lathered really, really well. I have used dual lye with around 5% KoH and 95% Naoh, and there is definately no lack of lather. I have used coconut oil also (not sure about the percentage either, since it was in the same product, and sometimes I added a little extra coconut, but the coconut percentage have never been high) and castor oil at around 7-9%.
If butters don't lather, it is very easy to make them lather. So I don't think that should be a big problem, since it can easily be fixed with especially dual lye. You can also use additives like sugar dissolved in the the water before you add lye to boost the bubbles a bit. Or blend a sugar solution into the oils. You can increase the coconut oil and castor oil a bit, if necessary and if wanted. They say castor oil will create a draggy feel to the soap if used in a high percentage. I think that is true, I had one soap with a drag. But, I have found that kaolin clay counteracts that drag and make it work out just beautiful with a higher percentage of castor oil (kaolin clay makes slip, and that will balance out any drag, in my experience. But I'm not an experienced soaper at all, just sayin'. So others, more experienced ones will perhaps have very different experiences).
Are butters needed? Probably not. I think they are a lot about label appeal. I used a mixed product high in shea butter mainly because I had no other hard oils available. Yes, it was exactly that. That was the only thing I could buy in grocery stores other than coconut oil and soft oils. So I felt I had no choice. I'm not sure I would have gone online and order shea butter if I had cheaper alternatives locally, like palm oil or something. I have switched to soy wax, but I can't say anything good or bad about it, since my first soap with it is not cut yet. But it is a cheaper alternative to butters, and many soapers prefer to use it.