Even plastic tanks can benefit. Sometimes you can get a jelly sort of stuff build up in them, or you get the likes of the Cagiva Raptor, where its plastic tank has a bolt in fuel pump assembly with exposed steel fuel pump wiring. In a triumphant masterstroke of engineering design, those terminals sit at the lowest point of the tank. Right where the water collects. Genius. Luckily you can get the complete plate assembly from Suzuki as a TL1000R part - at around $800 (from memory) it's about half the price of the probably now non existant Cagiva part.
Another thing I see with plastic tanks is their cap recess drains blocking at the little screw in aluminium barbs in the bottoms of the tanks. Often the rubber hose routing after the barb is such that there is an upward run causing water to collect in the barb. They just fill with white corrosion, which can go surprisingly hard. I find the easiest way to clear them is to run a 2.5mm drill through them, which also illustrates how small and easily blocked the hole is. Best to remove them before you go drilling into them while they're mounted in a plastic tank, but then again I've had a few break off before coming out. I now keep a couple on the shelf. Any of the late model bikes with plastic tanks can suffer from this, including Aprilia and Moto Guzzi.