Again: if you don't upgrade and are a solo miner, pool operator, or p2pool user you will almost certainly waste time hashing bad blocks after April 1.
This only applies when you include transactions generated by other people (because the old client will not be able to fully verify them).
If you think this statement is incorrect, then please elaborate (with technical details).
That statement is incorrect.
There are two ways you might waste time hashing:
1) Put a bad BIP16 transaction in your block
2) Building on top of a bad block produced by somebody else
So even if you don't include anybody else's transactions in your blocks you will still almost certainly waste some time hashing by building on top of invalid blocks produced and announced by some other lazy miner running an old version of bitcoind.