If someone's handwriting is illegible it's because it's messy - and that's not necessarily linked to whether the letters are joined up. I've seen messy examples of both. It's not like children spend ages learning it either.There’s a lot of things in primary school which are “taught” but I deem useless. Joined up writing is just one of them. I feel writing in a style that is easy to read is more important. Certain children just do not get the hang of joined up lettering no matter how much it is taught, especially those with naturally poor fine motor skills. Some of these kids actually end up excelling in other areas but feel like a failure because their handwriting is constantly critiqued. Who gives a toss if it is joined or not as long as they understand what they are printing on paper?
Additionally, the fact I (and many others) still join their letters when they write shows it wasn't useless for us all.