You certainly could have a more nuanced policy, and use more images as a result!
I'd guess the reasons you might be cautious are:
- Commons has a good number of files it that are in the public domain in the US, but may not be in the UK (assuming that public domain-UK is sufficient for the BBC to make use of something)
- Commons also has a good number of files which we believe are public domain (at least in the US) but where someone still asserts copyright from "sweat of the brow" rights (this set overlapping with the previous one a fair bit)
- Some images might simply have been mis-attributed by whoever uploaded them to Commons and are actually in copyright (probably a much smaller group than the first 2)
Of course, these kinds of issues aren't unique to Wikimedia Commons, any open image source could have the same problems - so if you are taking PD images from Flickr or elsewhere on the Internet, then you shouldn't have a blanket bank on Wikimedia Commons!
Paid photography sources potentially have the opposite problem, in that you can end up paying royalties for images that you think actually are in the public domain.
As Charles says, it's usually possible to come to an informed judgement based on what the Commons page for a given file says, but this needs a reasonable level of awareness of copyright law. I'm not sure if there is any kind of "user-friendly" summary at the minute, but there probably should be - I'm asking around...
Hope this helps,