delete-old issue with src.opts.mk

The other night I built a new -CURRENT at r265728. At delete-old it asked
me if I wanted to "remove /usr/share/mk/src.opts.mk?" I said yes.
Subsequently an upgrade attempt for virtualbox-ose-4.3.10_1 failed with a
"missing src.opts.mk <http://src.opt.mk>" error.

Last night I built a new -CURRENT at 265793. At delete-old it asked me if I
wanted to "remove /usr/share/mk/src.opts.mk?"again. I said no. Subsequently
an upgrade for virtualbox-ose-4.3.10_1 succeded.

Three questions:
1. Why is delete-old repeatedly asking me if I want to remove /usr/share/mk/
src.opts.mk?
2. Why is delete-old asking me if I want to remove something that is
required to build ports, well at least one port? I did not try to build any
other ports, so maybe it was just a virtualbox-ose issue?
3. Should I keep src.opts.mk at delete-old or remove it?

I saw some discussion about src.opts.mk, but it centered around the use of
WITHOUT_NIS, which is not my issue.

Thanks.

-Tom