Also, you can have a much greater range of possible OG with different combinations of LME and DME just by using the standard packages. With a kit, I can't imagine NB wants to start repackaging in odd-size containers, so they're limited to 3 and 6# containers of LME and standard 1 and 3# packages of DME. It gives you a greater ability to dial in your target OG by mixing the two.
For example, you could have a target OG of 5 gallons wort of 1.052 using two standard 3# packages of DME. By using 2 3# packages of LME, you could have 5 gallons at 1.043. If you wanted something in the middle, 3# of LME and 3# of DME would give you 5 gallons at 1.047. Want an OG of 1.060? 2 3# packages of DME plus a 1# package of DME would get you there.