It makes no difference to me whether a plant is potted or not. Stemplants primarily absorb nutrients via their leaves through the water column, so buying a bunch with roots isn't essential. They develop roots as a means of anchorage and as a secondary system for nutrient absorbtion. That is why you can sometimes get a brand new plant with a single leaf. As long as the stem is healthy and given proper care, the plant will do well. Some stemplants have pronounced roots in the water column (Brazilian pennywort), but this particular stem plant is predominantly a floating plant, so as with most floaters, the roots will appear in the water column.
Root feeders (crypts, etc) feed primary through their roots in the substrate, and they can absorb nutrients throught the water column as well, but that is their secondary feeding method. They are often potted to protect the roots during shipment, though I have also received these types of plants bareroot as well, with no difference in health. Again, as long as the roots are light-colored (creamy white in crypts, yellowish in marsilea and other rootfeeders, often with green growing tips or paler) and have a firm texture, the plant is good to go.
Many times the air circulation in the potting medium in potted plants is very poor. Plants need circulation, like anything else and that is sometimes why roots and stems in that medium can appear black or brown. These damaged portions should be removed immediately as they are dead or very nearly so.
I purchase what is cheapest and usually non-potted plants tend to be.
