Perennial rocket

I keep reading about people digging up their rocket when it goes to seed, but I have had the same 2 rocket bushes in the back garden for about 5 years.  I cut them down a bit and squeeze them into a couple of cloches in the late. Autumn and take them off in march and we eat the leaves all year.  They just get more and more peppery!  I cut the flowers off and we eat those too, and the bushes are quite big ( yep, def bushes now, couldn't really call them planrts). Why do people pull them up?