Honestly, I don't really understand when people change their listening habits because they want it to look good on their account. Personally, I've never fallen victim to that, and I've been using last.fm for five years now. It's just a fun thing to keep track of your listening history and statistics and to discover new bands/types of music. I usually scrobble everything because I like to keep track of accurate records, but it isn't that big of a deal. It takes me like 10 seconds at the end of the day to make a playlist of songs I didn't scrobble. The coolest feature for me is being able to see what I listened to at any point in time. It's kind of cool to see how your musical tastes have changed and progressed. That's really the whole point of last.fm for me, so I don't know why anyone would reset their last.fm, but if they want to, thats entirely up to them.
It's all fun. If you can't handle it without worrying what artists are getting too many/too few plays, it might not be for you.