Toxotai are the archers in Hellenistic armies. They make adequate archers, but cannot compare to those from the east and south. They, like other psiloi, are vulnerable in hand-to-hand combat, especially against cavalry.
Toxotai are the third branch of the Psiloi, the archers of Hellenic and Makedonian armies. They are generally from the upper end of the poor and recruited from mountainous regions where the use of the bow is an essential skill to keep one’s flock of sheep safe from roving predators. Toxotai are well trained in a manner of speaking, that being that they are using their weapon of choice (often of necessity) from birth. They are decent archers, but are nowhere near as professional as the archers from the east and south. They mostly use the short bow, which means that they are often outranged by their counterparts from other lands. This reflects their secondary role in a Hellenic army. As most missile units, they will be cut to ribbons in melee, so they should be well protected from enemy ranks.
Historically, the Hellenes did not use archers in any significant fashion. They did not have the composite bows of their neighbours, and their lands were not particularly suited to the cattle and horse farming that supplied the raw material for these bows. Therefore, archers fulfilled the same role as other Psiloi: that of long range harassment. The Hellenes and Makedones had no real tradition of archers and could not recruit any but these shepherds to do this work for them, since they lacked any access to the archers of the east. Most Hellenic states relied on Skythian and most importantly Kretan archers to do this for them.
Despite being the weakest archer unit in the game, with inferior range, ammunition, and unit size, as well as being very weak in melee, Toxotai have their uses. For once, they make cheap garrison troops. They are also useful for weakening unarmoured units, as well as using fire arrows to torch enemy siege equipment and inflict morale damage on enemy units, particularly elephants and scythed chariots.
However, if you can afford it, you should try to field better archers, and relegate your Toxotai to garrison duty.