Mal, IIRC, we tried it that way a couple of years ago.
I don't quite remember who all was complaining, but it wasn't me. I liked the other way.
An example, I'm watching the House Justice Committee w/Holder and during the voting breaks, even though I haven't refreshed my browser, if I click on a link to the ATF F&F thread, I have to scroll backwords to view the posts that were made, while I was watching.
I would much prefer to click on the last viewed button and actually be taken to my last viewed post in a thread.
It is no real bother to click on the "Mark This Forum Read" from the Forum Tools drop down, when any forum gets more unread posts than I care to read.
Added Thought: Yes, it may take a few weeks for everyone to get acclimatized, but this happens whenever change is introduced, whether it is this option or an upgrade to the forum software itself.