> Well, using a blockchain just for timestamping is pretty much worthless in practice.
Quite the opposite. Using blockchain for anything but timestamping is
worthless, because there are faster and less resource-intensive protocols for
about everything. Data storage, for instance, has distributed hash table, and
change coordination has leader election or consensus protocols.
> Bitcoin, Ethereum and others don't use a blockchain just for timestamping either, but store all sorts of data in it (transactions in Bitcoin and even complete programs in Ethereum).
You're putting the cart before the horse. These "all sorts of data" in
blockchain are in fact documents that get timestamped -- the timestamp
says "document X was issued before document Y". A Bitcoin transaction in
isolation from all the other transactions is nothing more than a semi-random
string of bytes. For it to be a money-like thing you need to have previous
transactions (and how do you know they're previous? by timestamp) that
testify the account had enough resources for all the output.