Sure, it's a great way to ensure you make it to the Elims! Except for Championship events, the 3rd robot on an alliance frequently plays a defensive role. And, many times robots are relegated to defense because their offensive value is weak. I suspect you'd be a top defensive choice if you could also climb a rope.
The only disadvantage to building defensive robots, in my opinion, is you usually end up getting selected by seeds 6-8. Reason being seeds 6-8 will generally try to choose the best scorer available along with the best defender available to neutralize seeds 1-3.
Of course all that goes out the window if you seed in the top 5

, but that's usually tough to do when your primary function is not scoring.