The pBI121 provides the backbone for Tsuyoshi Nakagawa's pGWB Gateway binary series, which also give very low yield in E. coli extractions. Actually I didn't care much about it in the past, because you just need a few molecules of plasmid to get Agro transformed. The vectors from Gent University (http://gateway.psb.ugent.be/search) give much higher yield but some overexpression plasmids in my hands also caused some trouble with artefact generation. Expression levels are quite high with those.