IDL's are fairly useless unless you're going to a country that enforces the use of IDL's for drivers.

Often these countries tend to be ones where English isn't spoken overly freely. Normally your driver's license from your home town and/or your passport is sufficient.

But as discussed earlier in this thread, if the county you visit requires the card, then get it. Will save many, many hassles with police and/or insurance if they happen.