Most likely, nothing bad will happen. However, you don't what will happen in future releases of Q2A.
Suppose in the installation process the amount of columns in the qa_posts table is checked and if it is not the expected amount the table is recreated as it is assumed q2a is not (properly) installed. If the core did that check after adding a column then the table would be recreated. This is clearly, a silly check. But you don't know what other seemingly silly checks might be done in the future that might break the core or, actully, a database upgrade.
So it isn't worth the risk to perform a "database core hack" while you can get away with creating a pluggable table that won't interfere with the core. And yes, if you asked me, I would sacrifice a bit of performance by doing the join in order to sleep well knowing I won't be responsible of anything bad that might happen to the users :)