Farms do not provide a lot of positives for your city. They do earn money for the city (only with "Rush Hour" installed) and provide a few low paying jobs. The biggest positive is the Farmer's Market and State Fair rewards. The Farmer's Market reward provides a demand cap relief of 20,000 RS and 150,000 RSS. You can bulldoze your farms, and still keep the Farmer's Market, it will not be as effective.
Farms & Pollution
Farms produce a lot of water pollution. Once water pollution arrives, you need to plant trees to help off set the effects of pollution. Farms near water will make the water turn brown. It's important to note that agriculture zones are not required for your city to grow.
A agriculture mod that will multiply jobs on the farming zones is available at SimTropolis. RCI farming ploppables can be downloaded at the same site.