OK, but "пръст", for example, has no tendency to be in any way lesser or shorter. It is just another vowel. In a song, one could sing it, пръ-ъ-ъст.
At least that is the case with modern Bulgarian.
It is interesting to look at the transliteration rules from English. Primarily, 'ъ" is used for closed 'u', as in Къстер (Custer). But it also indicates a half-sound, as in Картър (Carter) or even Бийтълз (Beatles). Note the ingenious way to transliterate an open 'e'.
Yes, I a agree wholeheartedly, Bulgarian transliteration is superior. Other Slavic Cyrillic languages would greatly profit from aodptiong the same convention but I think nationalist pride is a great obstacle