I don't because they can sometimes contain other substances that I either don't need or don't want, especially sweeteners of any kind. However, they may contain other electrolytes in quantities that might put my system in the hyper range which can bring problems of their own. I haven't investigated the comparative cost, but my guess is that a pill with 200 mg of whichever electrolyte you're short of every two/three days will be cheaper than a commercially prepared solution with its plastic/glass container, also a cost.
If you really do need to drink the solution, I would think about 100-200 mg per day, or every other day, so you have to read the labels and find out what the volume of each container is promising to give you.
Last point: ones kidneys are what are supposed to release overages of electrolytes, whether sodium, potassium, magnesium, or calcium. Kidneys that are in moderate or poor condition may struggle to regulate the concentrations of these in the blood and in the urine, so you should probably run your intentions past your physician to ensure you can be monitored for at least a few weeks to see how your system handles the additional consumption. Just a suggestion.