"How impotant are safty glasses when shooting?"
Only as important as your vision.
I've had hot brass eject back and bounce off my safety glasses. When wearing a borrowed set I've had it drop between the lense and my face. Shooting with safety glasses is important just to prevent the sharp edge of a hot case making an impression on your cornea. Imagine how much more important they are if your weaopon has a catastrophic failure (read BOOM!)
Here's the facts. You can't "guess" or "think" your glasses are good enough. Just because you're wearing polycarbonate lenses as thick as the bottom of a 1950's Coke bottle you have NO idea what impact they will or will not protect you from. The only way to have any idea is for there to have been some sort of scientific test that determines what they will protect you from.
That's where the ASSE/ANSI Z87.1 standard for eye protection that people here are referring to comes in. Since the protective eyewear standard came out in 1998 a lot of changes have occured in materials so the standard was revamped in 2003. One of the major changes was the introduction of the "high impact" category. Z87.1+ is the marking found on the new high impact type glasses. Here's an excerpt from a nice article explaining the changes.
Impact resistance
Perhaps the new standard's most significant change is the introduction of two categories of impact resistance: basic and high. In effect, this dual-tier approach will divide the eye protection world between non-plano (prescription) and plano products.
For non-plano lenses, the new standard still has thickness requirements for basic and high-impact products. Lens thickness in non-plano eyewear is an important factor for maximizing lens retention in frames during high-velocity impacts. For high-impact plano lenses, however, the thickness requirement has been eliminated.
In some cases, testing procedures have changed, with emphasis now placed on product performance instead of materials used. For example, the new High-Velocity Impact Test states that ". . . spectacles shall be capable of resisting an impact from a 6.35 mm (0.25 in.) diameter steel ball traveling at a velocity of 45.7m/s (150 ft/s)." Previously, spectacle lenses had to be at least 2.0 mm thick and resist such an impact. Now, spectacles with high-impact plano lenses need only to survive this and other impact and penetration tests.
Freed from the thickness requirement, manufacturers can explore the use of lighter, tougher materials. This is important because, though a 2.0 mm thickness does not cause unacceptable optical distortion for flat or slightly curved lenses, it does so in the sharper curves required for wraparound lenses.
The new standard will allow manufacturers to thin the lens material where the curve is sharpest. The result will be a thinner, lighter, distortion-free lens with better side protection. Because increased side protection is called for, the new impact testing occurs farther back (toward the ear) in the sideshield area of the eyewear.
New, even more fashionable eyewear styles are now possible. Look for increased use of wraparound designs.
New markings
Perhaps the most visible change will be the way that Z87.1-2003-compliant plano eye protection products will be marked. As with the old standard, all removable lenses, both plano and non-plano, must bear a permanent manufacturer's logo or mark. A brand new requirement is that high-impact lenses will be marked with a plus sign (+).
One-piece plano spectacles, like typical plant visitor eyewear, require only one mark: either "Z87" or "Z87+", whichever is applicable. This mark can appear on the frame, temple or lens.
Here's where the new standard may have some safety professionals scratching their heads. They may wonder how to tell the difference between an old-standard Z87 protector and a new-standard Z87 protector. It's an important difference, because new-standard Z87 protectors offer only basic-impact protection.
Basic- and high-impact non-plano lenses must meet many of the same performance requirements as plano lenses. Basic-impact non-plano lenses need to bear only the manufacturer's mark or logo. High-impact non-plano lenses will be identified with a "+". Under the new standard, non-plano frames that meet Z87.1 performance requirements will bear the mark "Z87-2".
The new standard attempts to clarify possible confusion by requiring basic-impact protectors to bear warning labels ". . . to alert the user when the lens(es) of a protector meet(s) only the basic impact requirements. . ." The label or tag is to be removed only by the user. But after the warning label is gone, there may be no way to tell the difference.
Under the new standard, manufacturers who make plano protective eyewear will find little incentive to make basic-impact models, as workers tend to opt for the highest level of protection available. It is quite conceivable that Z87 plano products will become a thing of the past.