There are two reasons incandescant bulbs are not recommended. The first is pretty straightforward: Heat. Typically you have to use fairly high wattage bulbs to get any plant to grow. High wattage (and even low wattage) incandescant bulbs create a whole lot of heat. Depending where you live that may not be a problem, but my fish would fry in the summer where I live if I used an Incandescant bulb for my planted tank.
The second is a little more complicated and has to do with the photosynthetic process. In order for plants to create energy via photosynthesus they must absorb light of a specific color. It is not just light that causes them to grow, but a specific color of light (aka wavelength). See the linked chart
. Incandescant bulbs provide light of the color that meets the 630-670 region wavelengths, but not the much larger region between 400-460 nm. As a result, you are not truly providing the plant with a type of light that it can use fully and the plant will eventually suffer because of this. Some plants that can do OK on low light might be OK, but your swords will eventually die if you don't provide more suitable lighting.
Go to Walmart, they sell two packs of 20 watt 6500k compact flouresacnts for like 6 or 7 dollars.