People are accustomed to being assigned seats at weddings.
I'm not accustomed to having assigned seats. I've only been to one or two that did, and I thought they were strange and uncomfortable - like I was being forced on particular people.
But I grew up in a southern, immigrant family, and receptions were likely as not outdoor barbeques. The seating chart one was for someone from a wealthy northeast family, so their culture was very different in lots of ways. Perhaps the suggestion there is consider what your families are used to.
Ardenbird and I had a reception a lot like what you are describing. We didn't have assigned seating, and it worked out fine. Well, actually, we tried to have a reserved table for the wedding party, so we could get a chance to talk more with our best friends, but it didn't really work out that way because a few other friends sat down too, and we didn't feel like telling them to leave. People mingled, although we did introduce a few of them as we circulated. And everyone reported they had a good time.