I think there's a confusion in the original between translation and interpretation (exegesis). The latter is understanding what the Greek mean, the former is expressing that understanding under certain constraints. There is often an art to translation, because expressing what we understand in clear, concise language can be difficult. Thus, "I have been and continue to be crucified with Christ" is too ungainly to be a translation though I think it adequately expresses the realization and continuation of a state that the verb represent.
At any rate, for perfect (middle-)passives, I've been coming to the opinion that the resulting state is more in view (generally) than the anteriority of the act. So for Gal 2:20, I would prefer "I am crucified" as a translation, though "I have been crucified" is still acceptable in my opinion.
Stephen C. Carlson, Ph.D.