In the narrowest context of John 8:58 "I have been" may sound as a perfectly correct translation of egô eimi, but in the largest context of the Fourth Gospel which repeatedly uses egô eimi as a Christological formula a locally awkward "I am" is the formal price to pay for the overall consistency which is theologically more important. In such cases it seems obvious that the NWT deliberately misses the forest for the trees
Slightly related question, but yet not in some way, What do you feel about Beduhn's preferred translation of John 8:58, "I existed (or have been,) before Abraham was"...... His reasoning is as follows:
"On the matter of word order, normal english follows the structure we all learned in elementary school: subject + verb + object or predictae phrase. The order of the Greek in John 8:58 is: predicate phrase + subject + verb. So it is the most basic step of translation to move the predicate phrase "Before Abraham came to be" (prin abraam genesthai) from the beginning of the sentence to the end, after the subject and verb. Just as we do not say "John I am" or "hungry I am" or "first in line i am", so it is not proper English to say "Before Abraham came to be I Am". Yet all of the translations we are comparing, with exception of the LB, offer precisely this sort of mangled word order." (Beduhn p.105)
He is equally critical of the NWT here, he argues that "ego eimi" may have theological significance, but at John 8:58 it is really just one of the most basic pron-noun + verb combinations there is, anything else is read into it and is outside the concern of a translator. Not that theology is is outside of concern necessarily, but that here the most straight forward method works fine, so why add to it.