Why should I not put content in the development environment? I've previously been told that I should not put content on Pantheon in the development environment.

The reason was not explained.

Can anyone think of any reason that this should be advised?

I can see that pulling/pushing a lot of content form the /files directory could be and issue but is there any other reason that anyone can think of?
I think that either what you were told was not explained correctly, or that you misunderstood.

You can put content on development environments. And you should, to test.

But that content won't be migrated to production. If you want that content on production, you'll need to create it on production. Or, you can create a migration strategy (using the Migrate UI for example) to migrate content between environments. But this generally takes a good bit of work, and isn't usually done.