If you are replacing Facebook, it is necessary... I do believe it will detract from the comments however. You will also get those annoying folks asking you for re-steems, or follows, or upvotes... when you ignore them on the comments, they will plague you on the messaging (so maybe a block feature is needed also).
Maybe also an archive feature for messages, all messages will disappear in 7 days unless you archive it. This will keep that message alone in storage or for a year.
Just some thoughts... as I travel and email is cumbersome... facebook PMs help without giving away your telephone (text) number.