Maybe this will explain: All stories require the reader to identify with a protagonist character. In the case of Yaoi and shounen ai, because it is written for women, it is often the uke, or female role, that is most identified with. When the identifiable character is ambiguous, particularly for staunch fans of the Yaoi genre, the reader is left feeling disconnected from the story. Basically, it becomes a matter of the reader being mismatched with the genre. This makes the story less enjoyable for those readers.
But that's not always the case. A story that requires the protagonist to be identified with as part of the target group, then the story is lazily written (as is often the case in yaoi).
I'll label the characters as seme and uke as much as I want. I'm not the type to remember names I'm not familiar with in the first place so please fuck off. In shoujo, I always refer them as the guy, girl, side guy etc but is that a bad thing? Would you have something to complain? I think not.
I know that the terms "uke" and "seme" are important titles to describe which character is which, but could we throw out the labels and just appreciate that it's a simple to goodness love story? It's more of a slice of life kind of story, rather than a "get my emotions running" kind of story. Don't worry about who's who and just appreciate it for what it is.