It is 100% necessary, they get MBD without it which means their bones are deformed and twisted and they suffer a lot. You have to use UVB for the dragon to be able to use the calcium powder.
The best one is the ReptiSun 10.0 tube, don't use a coil bulb, you could also use a mercury vapor bulb which is heat and light combined. You can get an under counter fluorescent light fixture for $10 at Walmart and the bulbs are cheaper online. Also I don't know how you house your dragons but they often don't do well when housed together. Good luck!
And let's go a step further and say what KIND of UVB: You either need the Reptisun 10.0 TUBE or there is another one - someone help me out- I think it is called Arcadia? You can mount your Reptisun bulb in the cheap under cabinet fixture that Walmart sells for about $8.00. Also, a cheap place to buy this very important bulb is petmountain.com. Check them out.
You don't want ANY other brands and you don't want the CFL 'curly bulb'. The Reptisun 10.0 is the ONLY one known to not injure their eyes and give the proper UVB. It also needs to be mounted inside the viv, and your Beardie needs to be able to get close to it ( like between 6-8 inches). No lens over the bulb - it keeps the UVB from getting to the Beardie. Others (like Jess) have written guidelines that are published on this board. Also, there is a section called "Lighting", read it. Be aware that your Reptisun 10 .0 bulb will need to be changed out every 6 months. The bulb will still light, but at that point, it is no longer giving off the proper UVB (you cannot see UVB) for the Beardie. Hope this helps!