Don't get hosed. Potential savings on water bills.

[Jeff Rabinowitz]

If your property is in an area where the water and sewer charges are billed separately and you spend a significant amount on outside watering you may be able to save some money by installing a separate meter on your sprinkler system. The water use associated with the outdoor watering may not be subject to the sewer charge. Of course, any savings will depend on the minimum charge that would be applied to the extra account throughout the year and the policies of your local water utility so check first.

Also, if you experience a break in an outside pipe (those of you in areas where pipes freeze in the winter probably know what I am referring to) the water utility may refund the portion of the bill they sent you if you ask and/or file a claim. The bill from a broken line may reach into the thousands of dollars if the water is not shut off quickly (how quickly will your tenants notify you?). The refund of the sewer charge could help significantly.

[JD Martin]

It might make sense depending on whether a separate connection is free, and if you use enough water and/or your sewer charges are so high so as to make it financially feasible long-term. My experience, and I have vast experience in this industry, is that the only people that can make the numbers make sense are commercial outfits (i.e. nurseries, golf courses) or large-scale housing projects that have a lot of grounds to water. Unless your utility will make an irrigation tap for you for free, you'll likely never recover the money you spent on the tap on sewer savings. 

Most utilities will have some kind of cap on the water leak overage credit, with 50% being typical.