There's a risk that things like fungus will be passed between your plants, a fairly high one. If your enclosure is free of pathogens to begin with, it won't be a problem, but if there is something there, the water will help a lot to transmit it. Also, certain plants like Nepenthes generally don't like to be in standing water - their pots can be in water, but the roots generally shouldn't be submerged. I think some other CPs are finicky about standing water, but none come to mind. I don't think you'll run into many problems, as long as you have good ventilation and your plants are healthy (and appropriate for growing in standing water.)
Changing it every week may not be necessary. One option is to add an aquarium air pump and an airstone to the water - this will keep the water aerated and circulating, which cuts down on the growth of algae and other nasty stuff, and the bubbles will also raise the humidity in your tank.
~Joe