Title asks all.

Why might it be a bad idea?

The chance might you download an image claiming to be something else, and discover that it's porn.

Always the chance of that happening unfortunately. Spam-BOTs will return results that are similar or replicate your search terms. If you wished a greater opportunity to avoid spam-bots, try WireShare instead. WireShare has some in-built mechanisms to block out a few specific varieties of bot including the most common Spam-BOT.

Alternatively, you can learn to avoid the image spam. Along the way you start to recognise which search results are inevitably spam. For All Types search, you will notice spam tends to show up in each of image & video (& possibly audio.) The trend and file sizes are generally the same or similar. Likewise any oddities in regards to how the files are named.