On the flex issue, are you holding the poles correctly? Look at the picture on this site.

http://www.personal.dundee.ac.uk/~pjclinch/poles2.htm#Features

The weight should be held entirely by the strap, in fact you should be able to open your hand while supporting all your weight. The only reason you really need grip the pole is to lift it up off the ground. If you grip the handle too tightly (as in trying to use it like a staff to hold your weight), it puts side loads on the poles which cause them to flex, not to mention your hand will get tired and sweaty. If you let your weight hang off the strap, all the weight goes straight down the poles with no flexing.

Also, I have found the rubber tips aren't very useful outdoors in most terrain. Maybe on dry asphalt or clean, dry rocks, but any water or dirt will cause them to slip. The carbide tips give much better traction on most trails, especially ones with wet rocks or sand covered rocks.

My trekking pole is a Black Diamond as well. The locks have held up quite well considering how I abuse them. Note that it is possible to crack the plastic by tightening the screw way too much.

When going downhill, I think you have to grip the handle firmly to get any real benefit. At least that's been my experience.

I know cross-country ski poles are designed so the strap takes most of the force, but they are very long and so push you from behind.

When walking in rough terrain I need to brace myself from a full range of angles.

For a walking pole, I remove the strap entirely. I think it's a bloody nuisance, but that's just me.

I also remove the rubber/urethane tip when walking. It's great for the car and for shipping the pole in luggage. The carbide tips can be sharpened with a diamond file or a small grinder like a Dremel.

If you like to use the rubber feet, be sure and carry spares.

I've left several in various mudholes around the Colorado mountains when they get sucked off the end of my Tracks staff. And for wear-thru.

For my pole I've found chair leg tips of the proper inner diameter work fine. Buy 'em by the bag :-)