That's not correct. The Basic Multilingual Plane (like Unicode itself) is not an encoding, it is a subset of the Unicode codespace. UTF-8 restricted to sequences of 3 bytes can only represent characters between U+0000 and U+FFFF inclusive, which happens to exactly contain the characters defined by the BMP.

Most smileys now ubiquitous on smart phone keyboards are valid Unicode characters outside of the BMP set, yet being used constantly on the web. BMP just doesn't make sense anymore, nor does mysql utf8.