Is having an ‘OR’ in an INNER JOIN condition a bad idea?

This kind of JOIN is not optimizable to a HASH JOIN or a MERGE JOIN.

It can be expressed as a concatenation of two resultsets:

SELECT  *
FROM    maintable m
JOIN    othertable o
ON      o.parentId = m.id
UNION
SELECT  *
FROM    maintable m
JOIN    othertable o
ON      o.id = m.parentId

, each of them being an equijoin, however, SQL Server‘s optimizer is not smart enough to see it in the query you wrote (though they are logically equivalent).