The company explained the clean-up as part of an effort to "present more accurate, credible information" that people can rely on. While it didn't elaborate on what that meant, this could help you find the users and tweets you're looking for instead of having to wade through a sea of quickly-abandoned accounts. This could also make it harder for porn merchants and propagandists to hijack accounts for the sake of spam.
This could help if you're frustrated that someone grabbed a coveted username years ago. However, there may be a significant downside for anyone who can't sign in. The deceased, people in extended hospital stays and others might lose their accounts, and thus their entire post histories. You might not revisit a loved one's post history without delving through archived pages. This could also pose problems for well-known accounts that have fallen silent but still have some cultural value.
We've asked Twitter if it can elaborate on how it will handle idle accounts. It's safe to presume there will be many people hunting for newly liberated usernames in December, though. If you've coveted a username for a long time, you'll want to watch closely in case it opens up.
Update 7:17PM ET: Twitter told Engadget it's considering ways to memorialize accounts of the deceased, though it didn't explain how that might work.