Stumbled again over this: Until today i haven't used the core shared events, nor my users in their "updates" pages. The updates pages are completely removed. From the beginning I was using Noah's history plugin, then the q2apro onsite notifications plugin to write events into the eventlogger table. From this table I am reading all data and displaying it frontend, each user has a 7-day-history page.
I strongly suggest (since my db qa_sharedevents already reached 80 MB and others are growing even more ... for nothing), to provide a switch to turn off the "event trackings" individually.
Or only rely on the event logger which is doing an awesome job in my opinion.
EDIT: I am thinking if I can write a plugin that overrides function qa_db_event_create_for_entity() with a simple return;
PS: From http://www.question2answer.org/secure.php "To save space and time, you need not back up the following tables: qa_cache, qa_contentwords, qa_iplimits, qa_posttags, qa_sharedevents, qa_tagwords, qa_titlewords, qa_userevents, qa_userlimits, qa_words. These contain information which is temporary or which can be recalculated from other tables in the database." --- So I guess qa_sharedevents and qa_userevents can be neglected.