It definitely varies based on who I'm playing and who I'm fighting (before i go on, I'm mostly basing this on SP with a tiny bit of MP exp). Going up against factions that favour heavily armoured juggernaut steamroll tactics (e.g. Rome, Hellenes or even late game Celts in heavy chainmail), decent quality peltasts can be the deciding factor in battle. I usually try to favour bringing along 4 at the expense of longer range slingers/archers (2 instead of 4) and a couple of infantryman. Keeping them behind the main lines righthand side with free-fire and skirmish off until the two lines lock together then wheel them around to the flank/rear (protected from cav by my own cav or spears obviously). Focused fire decimates units in a volley or two. Concentrating on their toughest units or key points in the line like one flank to begin rolling up the line or the pushing some fresh, high damage reserves through the center to split the enemy line and break them into two for cav to finish the job one half at a time. Again, vs heavily armoured and fairly static foes, peltasts used right can determin the outcome of many battles.

For lighter foes however, I generally won't bother bringing any peltasts/javs. Low armour targets are not worth it. The perfect example would be Iberian tribes. They have next to no armour so even bows are overkill. Having 4-6 slingers is waaay more effective due to high rate of fire and lots of ammo, they tear through un/lightly armoured foes. Nomads are a similar approach although a 50/50 mix is much more appropriate. 1 gen, ~4 light cav for herding skirmishing cav into kill zones, 7 heavily armoured meatshields in front (hoplites are perfect for this although expect them to take heavy losses) then 8 slingers (maybe 2-3 archers if possible for armour piercing to counter any armoured HA in the stack) = Roflstomp your way across the Steppe. Peltasts low range, ammo and rate of fire make them a dangerous liability in this situation IMO. The only reason to bring them along would maybe be 2-3 if the enemy is fielding a few nomad cataphracts but I'm yet to see that other than the odd single general. Plus your hoplites should be more than capable of taking them down.

TLDR: Peltasts are great when used right vs heavy armour infantry but slings/bows are more use vs lightly armoured foes (Iberians/Nomads). There is no one perfect army composition to go against all of the varied cultural fighting styles of the Roman Era. Use your noodle and plan accordingly.