Any TII engine, even seized, has several unique parts that make it worthwhile to retain. They won't get more available, ever.
OTOH, a "plain old 2.8" has next to nothing, other than a matching number, to distinguish it. So the value is only that... the matching number, so keep the block if you're enthused to. As Teahead suggests, it'll never be worth anything except to the next owner, and chances of that are marginal at best.
Who, ever, would replace a S38 with a M30B28, even when/if the former has croaked? I don't see anywhere that the "matching numbers" craze has hit the BMW market.
Still and all, I have the auto trans. torque converter and flywheel that I took out of my car after it wouldn't find reverse, and I freely admit it has negligible ongoing reason to exist, only that it's a heavy lump and deeply darkly buried, as well as being difficult to legally dispose.