I found the scythed chariots useful for the Seleucids for a little while - blacksmiths are available much earlier than the buildings you need for your half-decent units, and the chariots dont use many men, so there isn't a huge hit on population.

When I had them, I used them either as shock troops - get in and target any missile or cavalry troops long before the rest of my army arrived, so that, hopefully, they're still fleeing/routing when the big boys turn up. That way, if (when!) they run amok, they're a long way from your army and continue to keep the enemy occupied while you march up. If they do run, they're fast enough to flee the battlefield and survive to be retrained another day.

The other use was to just get an army of, say, 6-10 chariot units and occassionally an inexperienced general, and just send them off on the map in advance of the main army. Any one they ran into, just send them in! They delay the opponent, can completely rout any weaker units, and will usually do a few hundred casualties to a large army, before running away to fight another day. It makes a perfect army to clear peasants away as well!

Once you get decent elephants, cataphracts and the rest, don't look at them again, but early on, they're the best the Seleucids have. Check out the 'guides' section for more details on the different factions' chariots - there are more details in the Seleucid and Brit guides.