Just to make sure that we are talking about the same thing: I assume that you need to know all transactions that you include into your block before you start the proof of work. Please correct me, if I'm wrong. So I do not see the point of wasted work.
The whole point of the block is to secure the transactions in it by piling proof of work on top of the transactions. Before a work unit can be assigned to a mining program, the previous block and the set of transactions to be included in the generated block must be known.
Generally, if a new transaction is received, it will be included in future work units assigned to miners. Past work units are still valid, and if they result in mining a block, the new transactions will not be in it. This means that when assigning work units, a record must be kept of each work unit assigned to a miner to allow the correct set of transactions to be bundled in the block should the miner succeed.
Also, as long as I'm responding to this thread, I should point out that long-term, every miner has an investment in the bitcoin system itself. If miners don't include transactions in their blocks, the value of the bitcoin system as a whole is reduced. Anyone who mines a block today has a good chance of mining blocks in the future and has a stake in maintaining the future value and stability of the system as a whole.