MCC is a hex dump that most players, and people, cannot read. Plain text opens on every device and is easy to search or paste elsewhere. MCC is at home in closed captions for broadcast and streaming delivery that carry CEA-708 as well as CEA-608; TXT is the usual choice for notes, logs and text that must be read by anything.
The text is read as UTF-8 or UTF-16, whichever the file is. An older file in another code page is read using the charset it declares, or as Windows-1252 if it declares none, so accented letters in an unlabelled legacy file can come out wrong. A file that is not text, such as the picture-based subtitles of a DVD, is refused instead of being converted to nonsense. The CEA-608 caption data inside each packet is read, as for SCC. Captions that exist only as CEA-708 service data are not read. The result is a transcript with one cue per line, without times; a cue's own line breaks become spaces.