We'll use Spit IV for the basis of this comparison.
(150*2)+(1500*2) = 3300. A decent whack of DMG but still acceptable.
(150*3)+(1500*3) = 4950. Possibly kill a non-HP stacking Demigod.
(150*4)+(1500*4) = 6600. Only a HP-stacking Demigod would likely survive.
(150*5)+(1500*5) = 8250. Well above what I would call acceptable.
Keep in mind that this DMG is at range, is dealt mostly over time thus allowing the attackers to retreat and can be pumped out every 7 seconds assuming there are no cooldown reducing or increasing effects in play at that time.
Spit's strength is it's overall damage. However, its weaknesses are that it has no special effects, offers little instant gratification, and has a chance to be removed and therefore prevented entirely.
We can work out similar data for any other combination of 2-5 nukes, be they duplicated or diverse. The overall damage will be a bit less than mono-spit, but will offer other effects that in many cases will outshine mono-spit. Assuming level 4 skills:
Spit x 2 = 3300 damage
Spit + Penitence = 2618 damage (including Pen's effect on spit for the first 7s), plus 16% amplification of any other damage hitting the target, plus 16% snare of the target, plus a chance for an interrupt.
Spit x 3 = 4950 damage
Spit + Penitence + Bite = 3517 damage, 16% damage amplification, 46% snare, interrupt, -700 armor, and 775 life restored to LE.
And of course there are other factors to consider like the situational useful/unusefulness of harassing with ranged nukes, the possibility of effects being removed early by Heal 3, the influence of health regeneration and especially priest minions, etc. All in all I'd say the data shows that in most cases a diverse array of nukes not only competes with mono-spit, but is clearly superior.