When I play EB I intend to do it as historically as possible. I don't know how one can download and play a historical mod without a set of house rules on playing historically, of course :P, and that means recruiting hastati even though they are completely useless when down to the stats. Besides triarii are expensive and intended to be rare, since they are more of an elite unit proper than some manstay of every army: you should use principes for that, and considering the fact that Romani is one of the factions that can field tons of armies compared to less privileged factions such as Arverni fielding lots of elite units gets old quick. Besides, the use of a formation such as the Quicunx can make otherwise lame units to be very good and less costly than the whole triarii option.

As for the spearmen question, it is indeed true that Polybians suffer against cavalry. But then the main enemy here is infantry, and Polybians are the ones who excel at fighting them: usually you will still be facing hordes of Greek hoplites and barbarian infantry, coupled with Carthaginian phalanxes and heavy infantry. Polybians are the RIGHT guys to face them, while enemy cavalry is more of an afterthought: usually your triarii can protect your flanks, or some cheap levy unit like Iberian Spearmen if you don't expect heavy opposition. Against the Carthaginian bodyguards even Samnites do, as long as you employ them with local superiority in numbers and provide for some support in case things get dire, like a cavalry charge.

By the time you really get to the factions that employ cavalry heavy you're probably already on the Marian period, which means a wide AOR for heavy infantry, and a wide range of auxiliaries over the map. Like countless spearmen of diverse qualities to withstand the charge of even the fiercest Nomad cavalry, and as for HA's Marian units can actually use the Testudo while the Camillans can't.