Once it is bent you are creating a weak point where it is likely to bend/brake even further. If you are very careful with a pipe bender you can make smooth bends, but it is usually better to use a tubing bender and create smooth corners.
The structure is in the triangulation, the bends don't matter. A cheap harbor freight pipe bender is going to kink pipe when you do 90* bends, there's no way around it. But, it doesn't matter because the bends aren't structural. The bends exist to follow contours & to shape the cage & have minimal function in structural strength in a rollover event.
The main difference between Pipe and Tubing is the Tensile strength. DOM is around 70,000 and 4130 is around 90,000. Sched 40 is listed around 50,000 depending on the site (aka, that's what Google came up with).
And Sch 80 is 80,000 lbs - which is stronger than DOM or 4130 tube. So, use 2 sch 40 pipes, or one sch 80 in the area over your head (B-piller I think it's called) if you're worried about it. I'm not, as I've seen 4-runners, blazers and full sized bronkos with sch 40 pipe roll / flop and they're fine. Since I'm a good 2000 lbs lighter, I'm not in the least bit worried about it.
I've read that Pipe becomes more brittle then DOM after welding, but I don't know enough about metallurgy to give an opinion on that.
x2
But, again - since I've seen them hold up, I'm not worried about it in the slightest. 40,000 psi is a LOT of force. Far more than I'll ever put on it, so there's plenty of wriggle room to weaken things a bit with welds.
For the record, I may be an advocate of pipe if that's what someone can afford, but I'm not trying to tell people pipe is superior to tube. Tube is lighter, and will keep a lower center of gravity. But, being just plain out of budget for most people makes it useless to them (and me for that matter). So if pipe is IN budget, I hate to see people scared away from it when it will work very well, and is far more likely to keep someone alive / injury free than a stock Samurai roll bar, or in the case of kicks - no bar at all!