30-round mags are about as big as I want. Even they hang down too low for my tastes sometimes.
I currently have seven 30-rounders. The two new C-Products Bushmaster mags, along with a slightly used D&H (I think) Bushmaster mag, travel in the mag pouches with the carbine. These (especially the C-prod) are much "prettier" than the four new Center Industries mags that stay in the ammo can.
All of my 20-rounders (11) are refinished Vietnam-era Colt and Simmonds mags with new springs.
I certainly don't "need" a 40-round mag, and may not ever use it if I had one.
But as I said, if the C-products 40-rounders really work I just might break my self-imposed mag moratorium and get one.