I wouldn't say it's necessary
. Most/all of the set bonuses can be found on the wiki, and they are nonrandom, so anyone who would need to know them could just look up one of the items and find it there.
Example: Earring of Sacrifice
(You can see the set bonus right above the item info)
However, it never hurts to provide extra info on items, since it would save the trouble of searching the wiki. I'd see it better as a guideline/suggestion than a rule/requirement, though.