Why is it bad to use the GUID?

Question

I recently upgraded my WP version from 3.2.x to 3.5.x. In the process, the WP functions that retrieve images (wp_get_attachment_image_src, get_the_post_thumbnail, etc) are now broken. I had to change calls to those functions and use <img src='.$my_post->guid.'> in order to get it to work.

However, people have told me that using guid is not good. I'm not changing the value though, just using it because I can't find any other way to get my images to load properly. The wp_get_attachment_image_src method doesn't return anything. What's so bad about using the guid if it doesn't change? Is it okay in this situation?