Lol. Distilled water is not bad.
Depending on your growing style you may or may not desire dissolved minerals in your water source. Tap water will always have some dissolved mineral content. In hydroponics poor quality/hard water can lead to nutrient lockout and salt build up. In organics chlorinated tap water will kill off some of your micro-herd. Otherwise the dissolved minerals (calcium, magnesium, iron, et al) in the water may be beneficial and help meet those nutrient requirements.
If you're using distilled/RO water minerals can be supplemented; Cal-Mag for hydroponics and plenty of dolomite limestone in soil.
And tap water can contain a bunch of nasty stuff, it depends a lot where you live. West coast water for example can have a TDS upwards of 300 ppm, while my tap water typically reads around 70 ppm and distilled/RO water at or near 0 ppm.
http://water.epa.gov/drink/contaminants/index.cfm