There's not much point in knowing your blood type
It might save your life. If that's not point enough, I don't know what is.
Of course, in most situations where you need a blood transfusion you aren't able to say it yourself so you should probably have your close ones know it too and/or have it on a card in your wallet or something.
It wont save jack because in most situations you're not going to be transfused without at the very least a basic assay being performed, and in the ones they wont they will give you O neg while they do it. Certainly noone will stop to ask you your opinion (mind you, if the information they get from you is wrong THEY might be liable).
In facr, I daresay the reason why most people don't know their blood group is that there is not much point in them knowing their blood group. About the only time I can think we grouped otherwise healthy people was to keep a record of them in the database because of special requirements.
In a controlled medical setting, yes. Still. Knowing your blood type could save your life, or at the very least prevent nasty complications when you ever need a blood transfusion somewhere in abscence of testing equipment / staff.
... and what situation is that exactly? If there is no equipment or staff there is likely no blood either. You want to jury rig a transfusion in the wild? Mind you, it kinda can be done if you're really desperate but you'll want to hack a crossmatch anyway. I'll grant you that in this hypothethical Bear Gryllis scenario it would be kind of handy for everyone to know their blood group, but bear in mind you need to be far away enough for civilization for it to be necessary, yet to have enough equipment and technical skill to get it running. Not bloody likely! (pun very much intended)