And it's not just with guns. A tricked out 12th level character with a rapier can do 1d6+10, 17-20 crit. (It was a hypothetical build I made, a Strong4/Soldier8 duelist. Nasty.)
High-level characters are less likely to ignore squibs, because there's always the chance that the squib rolls for a crit. Heck, give the squib an assault rifle, and you don't even need a crit, just a good roll. Or give the squib a heavy handgun and the Double-Tap Feat. PCs are still heroic and badass and all that, but your katana guy doesn't just pose and ignore the eight guys firing at him with AK-47s, even if he's 10th level and they're Str1/Fst1 ordinaries. Or at least, he doesn't do it for long.
Grenades are VERY lethal. Which is good. I don't want everyone going "Oooh, a grenade, how scary, okay, 22 points damage, bummer, but no biggie," when one lands at their feet.
In a combat-intensive game, it ends up being a "but you could still die if they rolled well and you rolled badly" scenario for combat-ready characters (Strong, Tough levels), and a "really try to avoid getting hit, REALLY" scenario for noncombatants (that scientist with Int17 and Con9, for example). Again, very reasonable, very good. Everyone does fine, generally speaking, if they're about as cautious as their class levels indicate they ought to be. Tough heroes can wade into it a bit more, but Smart and Dedicated heroes should stay in the back ranks unless they've got a lot of tricks up their sleeves.