Okay I totally don't understand. If the block size is not a limit then why would anybody drop a tx that they had already paid the ECDSA price for?
They haven't paid the ECDSA price. The decision is "I know how big this transaction, how many OP_CHECKSIG opcodes I'll have to compute to verify it, and how much transaction fees it pays. Should I do the work of verifying it or should I just ignore it?"
@ribuck: yes, the UI would be much simpler, but internally the client needs a model of what the miners are accepting. Maybe a really simple internal model will work if the UI is really simple...