Yes, more manufacturers should make graphing calculators. Currently, the Texas Instruments line of calculators is the most popular on the market, with calculators such as the TI-84, TI-84 Plus, TI-89 Platinum and more that are staples of every classroom. While Texas Instruments and a few other companies make good calculators, diversifying the market would encourage healthy competition. With more manufacturers making calculators, companies would feel more pressure to make innovations in quality and ease of use in order to stay competitive. This would create more jobs for product designers and result in better quality calculators in the long term.
Yes, there is room for healthy competition in this arena, and more manufacturers should get into the game and start making graphing calculators. Right now the only company I can think of that makes these is TI. Having more manufacturers would likely bring down the price of these expensive calculators.
We do not need more device manufacturers to create graphing calculators. I believe graphing calculators are largely sold in the academic setting, but remain largely unused. While there are some professions (architects, scientists, etc.) that may require a graphing calculator, most people do not use them. I recall several college classes that required me to purchase a graphing calculator. However, those classes never even used the calculator. There are enough manufacturers to fulfill the current need for graphing calculators.
Graphing calculators are most often used in classroom setting, and it is much easier to teach a concept when every student uses the same device. If several device manufacturers make graphing calculators, it increases the likelihood that some students will bring in calculators other than the recommended model, making instruction time much less effective for the entire class.