[ThunderCactus]

If it's got a roller bearing on one end, then they're not stretching the ballscrew, but they're probably allowing it room to stretch. Unless you're accounting for that in your software, it'll result in small positioning errors. Our HAAS lathe would change about .003" over lunch as it cooled down.
The angular thrust bearings on the servo side are probably to help dampen vibration and possibly correct any servo misalignment.
I know some machines just use roller bearings on both ends of the ballscrew, but you don't want to buy those machines for serious work lol

I work on anything that has a fanuc controller

But even the 'smaller' mills need box ways if they're doing moderate work, like the DNM500
I ran a HAAS VF3 and new style VF4 for a few years, I can tell you how important it is to have rigidity in the machine. And it makes a big difference even when you're doing moderate machining of steel. I could flex the head of that HAAS just running a 1" indexable end mill at 2100rpm and 50ipm at .125" depth and 70% cutter immersion.

linear rails are for things like;
-making sure large overhead panels slide straight when there's only one actuator that's off-center
-for mounting controllers to so they can slide back and forth on a long bed lathe
-for doing really fast positioning but low load actions like positioning laser cutter heads, or drilling and tapping small parts

They're great for fast and accurate positioning, but they're just not meant to handle loads.
But they're great on those tiny little chinese machines, wouldn't trust anyone scraping those so the linear guides are an advantage. And you're not likely to be doing anything very heavy on them anyway