Is this part absolutely needed? I know the brake dust shields rot away in salt land and people run them without them fine. Supposedly this keeps water out of the back of the bearing, but wheel hubs should still be sealed anyway.
They are a pain to get on. When the Toyota dealer didn't get one on right after changing a wheel bearing, it popped off, destroying the tone ring on the CV Axle and the wheel speed sensor in less than a second. I suspect many have the same thoughts as you, as to whether they actually need to put them back on. I also suspect many just throw them away too. I'm sure everyone else will have their own opinion on this, they'll be here shortly to tell you theirs. I put them back in place, mainly because I don't like to leave parts out when I do a car repair. That being said, here's what I've done, and it makes putting that back in work better. I wrap the end of the axle in electrical tape (for clearance) and use the axle to pull the cap into place inside the bearing. Then I take the axle back out, unwrap the electrical tape and see that the speed sensor holes in the cap and steering knuckle are lined up before completing the final assembly. It's worked really well for me the 4 times I've changed out the wheel bearings on my 2009.
you can throw them away.. but it has a purpose otherwise Toyota could have saved hundreds of thousands not putting this part on.
