They reason that they are like that is because two of them are standalones, The expansion you can get for about 60AUD? (50 even :3!)

But you will want at least Factions or NF before it, way to much content to miss out on. (Like in factions, you pretty much get a PvPer's dreamboat of stuff to add to the PvP areas, sorta..., along with an up to standard storyline like NF) (NF Has the in-depth Hero system... and well... a bunch of other goodies ;])