Versatility, and where do you want to go from here? If you are happy launching the same three rockets with the same three SU motors, and you can cover the cost, why change?
But if you plan to expand into high power and build more versatile rockets, you will need a variety of motors. More thrust for windy days, more impulse for higher waivers, more thrust and more impulse for that Mach attempt.
Besides that, I think most people start to get bored doing the same thing over and over again, and like new challenges. Building motors is fun once you get used to it. And then it start to open up avenues into research motors, etc.
I have one word of advice, however. Since you are in DC, if you ever think you might join MDRA and go high power, then consider CTI instead of AT. Our field vendor exclusively sells CTI motors, and you can buy them on-site with no shipping and no HAZMAT fees.