an adequate fee one day is inadequate the next.
Not necessarily. An adequate fee of day 1 might be a higher than recommended fee of day two.
also anyone running an older wallet client, that client's default fee is probably wrong.
thats the source of confusion.
Therefore the source of the problem are the users who aren't willing to either update their clients or use Bitcoin properly (fee wise).
these "spam" tx produced fees, not exactly a bad thing for miners.
Zero-fee spam is pretty much useless these days. However, as there are some users transacting (or trying to transact) in the 1-10 satoshi/byte range, spamming in that one will have a impact on the network.