a question about warmongers

[Ereshkigal]

Per berserk rule, if they don't have any enemy in melee range, they hit each other? Seems so, but at this point i don't understand why anyone should choose the warmongers instead of the spears without the warchief.

[Lanz]

Correct. The typical way around this is to not place them within 2" of each other. Since you can measure your own melee range at any time, it's not hard to control where they end up.

[Northern Ronin]

As noted, Berserk can in fact cause some dissention in the ranks, and cause attacks at your own models (other members of their unit, an unlucky Legionnaire, an ARM 20 Carnivean convently placed near by). And it at times can be a risk. Most of the time, you remember you can measure your Melee Range at any time to keep friendlies out of that line of fire, and you fight back to back as such;
W = Warmonger E = Enemy C = Charge Target
E E E E
C W W C

The warmonger on the left charges the one at the left, the one on the right charges the enemy on the right, and you work your way through 6 guys and don't see your own, thus you cannot attack him. Charging next to a durable target is another way to avoid murdering your own stuff, either a near by enemy warjack / warbeast you're unlikely to kill (or something like the Book), or your own super high armor model if you're really being forced into tight quarters. Of course, the Warcheif is another option to nip the problem, 3 points to let you fight in narrow areas and get some health back.

The biggest thing is just don't clump. There's no reason to do it anyway, and it's just really easy to avoid.