Philosophical, mostly. Any non-philosophical justification I could give you would really be me just porting over my philosophical justification and pretending it's objective.

Example: "I don't want to wait forever while I transfer the extra 38mb over the network." (That's the size of a vendor/bundle for a new Rails app.) I have never actually compared transfer speed in each instance, so that'd just be a backport. ;)

That said, as far as philosophical objections go:

1. Checking in generated files is not best-practice. This feels the same to me.

2. I do a lot of development:

    $ ls src | wc -l
         107

Not all of those are Ruby projects, mind you, but I'm a member of 25 GitHub organizations and have ~100 repos on my personal account. That's a LOT of duplicate gem data.

3. Updating gem files in the repo obscures diffs. If I'm working on a feature branch, and I have 3 commits, and one of them is updating 3 gems, I have a few dozen or hundred files changed. I just want to see my changes, dammit! I guess this one can be construed as practical.