You ask whether there's a downside to using saliva as a lubricant. What makes it good for digestion makes it not so good as a lubricant: The enzymes that help break down food can be irritating to the delicate vulvar skin. As we lose estrogen, the vulvar tissue gets more fragile and delicate; what once was fine may become uncomfortable.
I also hear from many women that water-based lubricants don't last as long as they'd like them to; they prefer a silicone or water/silicone hybrid lubricant for staying power through more foreplay.
That said, if it works for you and your partner, you can keep using saliva for some or all lubrication. Just be aware of the potential for irritation, and wash with a warm cloth after sex to minimize exposure.