You indeed would probably get better value using them for single trips on AA such as BOS-JFK, or return AA trips by transferring them to Iberia (who don't do single trips on Avios).

However you need 13k Avios off peak to go between BOS and LHR each way. Or what you could do is buy a slightly more expensive World Traveller (economy / coach) ticket and use 13k Avios to upgrade to World Traveller Plus (premium economy). So for a similar sum to mentioned in your first post, you can use the Avios to get a bit more comfort, if all aligns well. But yes, if you are doing a flat return trip from Boston to Europe, in economy, planned well in advance, you best not use Avios for it. If it's an emergency one way flight, that may be a very different position.