No, you're not really missing anything -- but it's the same issue that attaches to any proprietary and/or DRMed format, whether it's Sony's or, say, eReader. Of course the proprietary
non-DRMed files can probably be converted to some more useful format, but the only options for DRMed ones are buy it again or break the DRM.
As to why anyone would buy such e-books, well, there are a number of reasons, and I suppose folks who find those reasons (or some combination of them) compelling enough buy them and folks who don't, don't.
One reason might be that it's the only way to legally get a copy of the book they want to e-read. Another might be a desire to encourage the availability of e-books in the future. Another might be a view that DRM is largely unavoidable for the present time, and the only way to change it is to show the Pubs who insist on it that people both want e-books and are willing to actually come by them honestly.
These are just
some possible reasons, and not intended as a complete list, nor do I necessarily find them compelling myself.
As for why folks would mess with them, even for free, well, unless you want to make a specific stand that DRM is so evil that you won't have anything to do with it at all (a not uncommon perspective), then you don't really
lose anything for taking free DRMed books. I mean, if you can't read the free book again in 10 years, what have you
really lost? You didn't pay anything for it in the first place!
Bottom line is, we each have to decide for ourselves where we want to stand on the issue.