Django/Python and Ruby/Rails present their own (sometimes very significant) opaque complexities. Being complex, they actually hide their deficiencies for a long time. Enough so that sometimes rolling your own thing is not a significant time sink in comparison.

This also alludes to the popularity of do-it-yourself microframeworks. If you're planning on using a microframework anyway, I'm not sure your argument really holds up. You can pretty much use whatever not-completely-marginal language you want and be just fine.

Honestly the only valid argument that I come up with is that you can't find enough programmers that know or are excited to know that language to join your team.