It is not needed, per se. It is possible that one could roll a CS board with great grouping at/near the Pentagram. In that case, Delirium serves only to provide CC and some amount of safety to the player and merc. On my board, and I would guess many/most, the act of tele'ing to/fighting with De Seis will pull/draw monsters to his seal, away from the Pentagram (again, if that is where your density is, like mine). By using Delirium (i.e. Confuse) at the pentagram, it pulls those monster back towards the center where they can be glitched. If it was not clear enough, monsters deep into both wings (in my case DeSeis and Infector) do not get glitched because they are outside of aggro range while tele'ing to the last seal.