Ok, first of all, as said by djdannyp, there is a big difference between Messenger Plus! and Windows Live Messenger. They are not the same thing...
Second, you need to upgrade because there are various security flaws fixed. Not upgrading often means that you are exposed to certain risks.
Messenger Plus! never ever had the ability to patch Windows Live Messenger (or MSN Messenger for that matter) so that you wouldn't have to upgrade.
Then there is a recent issue with a small protocol change inside Windows Live Messenger where contact sign in events aren't passed properly. That might the reason why you aren't seeing those specific events anymore in Messenger Plus!' event log...
Bottom line: always try to update, for your own good...
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