The short answer is yes.

Now do you need one on a stock car. Probably not. Do you need one if you run larger wheels. Yes. The larger you go the more important it is.

Do you need them if you plan to race, or driving very tight turns on a regular basis, absolutly.

Something you need to keep in mind. Yes everything is tied into the body very well. However the sheet metal is very then, and flexable. When its new its resiliant, but over time it stretches, warps, and cracks. Seen it happen before. However it will take time. The bar of course ties everything together and strengthes the mounts.

Strut bars... is that where Mummers go for drinks? 

Sorry, couldn't resist .

I thought they made the ride stiffer.. is that desirable? How much would front and back stiffen up a ride? would it make it uncomfortable at all?

A strut bar isn't going to stiffen a ride as far as bumps and such. It will stiffen up the handling as far as making turns. A rear sway bar could make the ride a bit stiffer in the rear as it dampens the up and down movment of the rear axle.

I ask as on SOME cars they are redundant and pointless, from a function stand point.
On OTHERS they are a WELL use of $$$, materials, and weight, for total functional useage.
Also, some BARS are pointless, as they are structurally weaker than what they are supposed to be bracing.  So, a long straw (PVC. . .) is about as functional, regardless if a real brace would do something or not on a particular vehicle type.

I was asking about xBs, specifically and directly.

It sounds like it makes SOME difference, but less than other suspension mods.  So, I'll put it on the "long-term" list and get other stuff first.

I am running the Hotchkis sways and that made the biggest difference in the handling. I think that the upper strut bar in the front will help some. Now is it a must, probably not. But it will do a little. So when you tie it all together, heck yeah it will help. I am going to do one. But have to powder coat it before it would ever make it on the xB. So I have been doing other things first and will get to that one at some point.

Now that I got some of the looks things out of the way I think I am gonna drop some cash on these. I been wanting to upgrade the handling. Originally it was the first thing I was going to do then got carried away with other stuff. However now that I have had plenty of time to test the limits of the xB with stock suspension I can't wait to see what these will do.

Ok the front suspensions on most new cars are made of struts. Struts are a shock, spring, control arm combo. They are bolted into "towers" on the body of the car. A strut bar bolts to the two struts to cut down on flex to help stability and control.

Sway bars are bascially springs. They bolt to the control arms on the front and the rear axle beam on the rear. The help control body roll or sway. The strenght of the bar is based on its size in diameter. The larger the diameter the more it counter acts sway.

Hope this helps.