There is no reason dev-python/pymongo should unconditionally depend on dev-db/mongodb. For all you know the actual database runs on an entirely different system. It does seem to want to start mongod in src_test() so it's OK to keep the dependency as DEPEND="test? ( dev-db/mongodb )" (but not in RDEPEND obviously).
13 Oct 2014; Jeroen Roovers <jer@gentoo.org> pymongo-2.7.2.ebuild: Marked ~hppa (bug #525246). Conditionally depend on dev-db/mongodb (bug #525330).