If you search for morelikethis:Art|Physics, you get all physics results: morelikethis gets a representative set of words from those articles and assigns some weight to them based on their frequency in the wiki, and there is no reason for those weights to be equal. If we want to show a somewhat diverse mix of tasks, we need a way to equalize the score contribution of different topics. (Not sure how many topic pairs are affected by this problem though. It is obviously language-dependent, and probably depends on the difficulty filter settings as well.)
ORES drafttopic search might have similar issues (the current plan is to implement it by using morelike search on the drafttopic parameters), so while fulltext-based morelike search might not be around for long, experimenting with solutions for this problem probably does have lasting value.