So if somebody is just after the 50 BTC and does not care about the network, would he get a (maybe very small) advantage to just ignore transactions altogether? Like:

 * If I learn about a transaction, just ignore it (saves me memory, and if I do not tell others about it, bandwith)
 * If I learn about a new block, I try to build a new block on it with just the 50 BTC transaction

I guess, this optimization is small.  But it would reduce the complexity (LOC) of my client, which may be useful for further optimizing it.

Would a minimum block size or a minimum required number of transactions per block be useful?