I have one Bioconductor package that I am currently responsible for. Each bi-annual release of Bioconductor requires testing and squashing errors, warnings and bugs in a given package. Doing this means being able to work with multiple versions of R and multiple versions of Bioconductor libraries on a single system (assuming that you do production work and development on the same machine, right?).
I really, really like RStudio as my working R environment, as some of you have read before.