I think for the most part I haven't entered the "credited as" names—and would never tag with them. Asking my music player to sort by artist and finding 6 different entries for Tchaikovsky (from all the different ways people transliterate his on different albums I own)—would not only have no value, it'd have negative value.
I agree it'd be silly to have only the last name. Not to mention, there are multiple composers named Bach, Mozart, etc. Putting just the last name in there would make a mess, for the vast majority of music players that don't understand MBIDs.
But, if you're already not putting what's printed on the album, what's the point? It's not useful anymore to people who want exactly what was printed (I'm not sure if this matters to anyone in classical?). Seems like you should just not enter a credited as at all.
As a side note, entering artists aliases like this seems like something the site is designed to prevent. To enter an artist, you type the name in to the artist field, then select an entry from the drop-down list that appears. When you do so—it overwrites what you typed in, putting the canonical name of the artist. And it does so in the "credited as" field as well. When the site discards the 20 characters of transliterated Cyrillic I carefully typed in from the cover, clearly the site does not want them! Typing it once was already a pain. grumble.