So I just finished leveling ARCANIST to 15 to be a Black Mage...

[Defteros]

snip

ACN cross class are more of a support-type.
Archer, instead, help you boost significantly your DPS, and prevent you from pulling 2/3 of a dungeon's trash mobs from the tank.

that aside.
if you have a Summoner/Scholar in your party, a BLM virus is less useful, so you rarely use it ( if the others know how and when to use it)
dunno if SMN/SCH have something like enhached E4E though, can't remember.

Archer Cross Class are
numerically less, but are 100 times more useful then the Arcanist Cross Class