If you have a wort chiller already then it wouldn't be any different employing it for a half batch as it would for a full batch. Charlie Papazian writes in The Complete Joy of Homebrewing, 3rd edition, "It is always important to cool the wort as quickly as possible, pitch the yeast and maintain sanitation." In general, sanitation is the biggest concern when cooling the wort, so if you are a stickler for the rules a wort chiller may be the best option for you no matter the size of the batch.

On the Other: Wort chillers a pricey investment

If you happen to be a beginner homebrew hobbyist and are just testing the waters by brewing half batches, then perhaps the ice bath method of cooling a pot would be the best for you. A wort chiller is an expensive item and isn't completely necessary for smaller projects. At Midwest Homebrewing and Winemaking Supplies the least expensive wort chiller begins at $64.95 and the prices can go up from there. There are alternatives to wort chillers that are quick and sanitary and should be looked at before making the investment.

Bottom Line

It is indeed nice to know that you can cool your wort down quickly and cleanly, but chillers are not the only other method for cooling wort. If a homebrewer is careful about their brewing environment and chooses to use other cooling methods the wort will still become a great batch of homebrew. If the brewer is a bit more serious about their brewing and can foresee a long term homebrew future, including stepping up to full batches, then by all means make the investment in a wort chiller. Just remember to keep the chiller just as sanitized as all of the other equipment that comes into contact with your wort.

Source:

The Complete Joy of Homebrewing 3rd Edition by Charlie Papazian

More Information:

Midwest Homebrewing and Winemaking Supplies