I would be inclined to say yes. Mephy is good but his price hike in CA2018 means he is no longer auto-include. Captain Smash is probably better in the role IMHO as you cannot Deny the Witch with stratagems. While Mehpy can power himself without CPs, there will be days when he just can't seem to cast anything or an opponent shuts him down with Denies or a Culexus.
If you have all 4 Assassins available, chances are you will find an effective target for one of them every game. Even if not, an Eversore charging into the enemy lines on T2 will usually make a mess of something and take some of the heat off Captain Smash and the Blood Angels.