Print your files and documents using only the black cartridge will certainly reduce the overall cost of printing. Whilst it requires you to buy black ink cartridges more often, this doesn't mean it'll bring much trouble with the expense. Just so you know, black ink tanks are less costly than those colour tanks, that's why it is more wise to get a spent black ink rather than any of the colour ones.
Econo-mode or draft or black-only printing, whatever you call it, they're all quite the same, a printer feature that restricts the machine to print only in black. It's economical indeed, especially for those who have a printer that can still work even with a single cartridge installed which is the black. Do you know why it's not ideal to print without the black cartridge and let the colour cartridges do the printing? Once you send print jobs to the machine and there's no black ink to pull through, the printer will make use of what's available, and will combine the three colors to create a dark ink equivalent in replace for black.
In the same manner, the drawback of using only the black may result to some odd quality prints, perhaps a bit dull or lacks vividness. Gray-scale printing is what they commonly called it, since the output in Econo-mode are less clear or more close to gray shade. For this reason, it is advisable to use black ink cartridge alone only if the file you're about to print is not that important or is not intended for office use. Surely you don't want your reports and graphs to look less professional than it should be.
HP and Epson printers are two of the most prominent brands that support black ink or Econo-mode printing. It lets you save inks/toners by printing in draft mode, as mentioned earlier. If your primary reason is to save your printer's consumables, toner-saving mode is expected to consume 40-50% less inks/toners with every print. That would be enough to extend the life of your cartridges or possible draw out more prints from them. Additionally, if you prefer more savings, why not opt for eco-friendly toner cartridges and ink cartridges which retailers are selling for half the price of original counterparts. Just don't use let the price tricked you, sometimes we're not aware that our eagerness to save compromises the quality we may get.