Can anyone think of a script that uses the altstack and can do something no other script can?
Some algorithms will be much more shorter using the altStack.
For example this script is only 10 bytes long
OP_TOALTSTACK
OP_TOALTSTACK
OP_TOALTSTACK
OP_HASH160
OP_FROMALTSTACK
OP_HASH160
OP_FROMALTSTACK
OP_HASH160
OP_FROMALTSTACK
OP_HASH160
and (I can be wrong, please correct me) replaces 4 values on the top of the stack with their hashes, leaving the original order.
Try to do the same without altStack in less than 10 bytes.
Update1: OK, may be it is possible to do with OP_2DUP and OP_2SWAP opcodes. You can try to find out your own examples for using altStack
Update2: Note, that bitcoin script language is not Turing-complete. Not all things are even possible with current opcode set