As others have commented... if you are that worried about a few MB, then it's time to upgrade to a slightly larger hosting plan, as you will use that fairly quickly in the short term. Now, I can see if one was talking attachment storage involving gigabytes of space.
As for disallowing attachments, for many niches that would be a total killer! You can always adjust the allowed size of attachments/images and with images set them to be resized if they are beyond a certain point. But again, if your site doesn't need attachments/images in the posts, simply turn that ability off for your users. Make them link to outside sources for it.