I would think that peltasts would be more important as in ancient times skirmishing was a MAJOR element of battle. The ability of a light quick screen to harass the enemy and draw off some of thier units was very important.

I think TW does a bit of a poor job with them since they are so important. However I still like to keep some on hand as a good balance. Most non-Roman units don't have a nice close artillery option, one that gives charging enemies a second thought before and after closing. Plus they can sneak around the back of an enemy and attack from their unprotected rear. They can also charge other artillery units and send them running and can chase down slower retreating enemies. Then as Simon Appleton says you can always sacrifice them to buy time against really powerful units.

So while they are not a main line or death dealing archer they are versatile have some use, (and are CHEAP).