The tread on a road bike's tires is really quite unimportant and purely cosmetic. Road bike tires have tread patterns because they sell better, not because they perform better.
Here's some questions to ask to decide about replacing a tire:
- Can you see any of the fabric?
- Are you getting flats more easily than you used to?
- Is any part of the tire bulging out or lumpy?
- Are there visible holes/gashes in the tire?
- Does the wear indicator indicate you should replace? (Some tires have dimples or grooves that are there to indicate when the tire should be replaced. Some have an alternate color of rubber underneath, and when that shows through the tire should be replaced)
Smooth tread is actually ideal for road conditions, but not commonly found on tires. If you're on hard surfaces such as pavement/asphalt, the tread doesn't help you and actually slightly reduces contact area and increases rolling resistance. On most road tires the minimal tread pattern is just cosmetic to make their customers feel better (since a smooth tire looks slippery).
See also: Sheldon Brown's comments on tire replacement and tread patterns
If you plan to ride through mud, sand or other soft surfaces, you need tread. So when you no longer have tread on your mountain bike or cyclocross bike's tires, it's time to replace them (unless you plan to only use them on the road).
