There's really 2 kinds of berber. Cheap public building/school house berber (ugh)... and gorgeous thick pile berber. Obviously, one is very cheap and one is very expensive.
I tend to love creamy wool berber... but even the expensive type is offputting to many, since they don't know how to care for it... and the last thing you want a prospective buyer to be is intimidated.
I'm with your agent, however... LOOKS sell. And if you have carpet that needs replacing (stains, holes, smell, etc.) it will lower the value & desireablility of your house 10 fold. Meanwhile that beautiful clean line of new carpet, combo'd with fresh paint... just SELLS. People see a blank canvas, and start arranging their furniture in their heads.
It's baffling to me... but I buy OLD houses. Fixer'upper / Major Restore type houses. I see what a house COULD be. Most people though, don't want to even think work. They want move-in-ready, and finis.