I'd say +3 weapons aren't just needed, they are inadequate.
Lets imagine things without magic weapons. A level 16 Fighter has 180 HP (15 for lvl 1, and 15*(6+5)), and with a Greatsword, he is AC 19 (Full plate+Defender). So what would an appropriate mook look like? AC 16 and 100 HP? That is someone HUGELY down in levels. And yet.
Average GWF Fighter damage per swing is (12*(8.3+5)+3*(16.7+5))/20=11.85. It'll take this damage oriented fighter about 9 hits (3 rounds) to kill a mook. Guys who show up in large groups. It will take the longsword+shield guy 11 attacks, or almost 4 rounds. That is a LOT of dice rolling to kill something pretty darn weak. Moving the 5th attack to lvl 15 or 16 AND giving out +3 weapons will bring the fighter's offense up to "able to kill something that should be a speedbump in under 2 rounds). Just giving out +3 weapons won't. Even for the guy with a greatsword.
With the current rules, save or die is the only sane strategy at high levels, unless adventure designers manage to hold the line and never throw in groups of monsters with more than 50hp even for adventures designed for level 16+ PCs. Note that Ogres have 59 hp while being challenge level 2. So, um. Yeah. Right. For reference, the level 16 mundanely equipped GWF needs about 4-5 swings to kill an Ogre. Huzzah.
Further, Fighter vs. Boss is futility itself. For real hilarity, map out a lvl 18 longsword+shield fighter against himself. With an averaging damage output under 20 damage/round, significantly absorbed by 10 points of regeneration a round, the fight'll last 15ish rounds of utter boredom. With that kind of survivability against weapons, high level Bosses will always be taken down by a SOD.
The outcome of Bounded Accuracy, namely putting all the scaling into HP, which don't protect you against spells, is REALLY REALLY CLEAR. Very long, very boring fights unless a spellcaster gets involved or seriously impressive gear (which breaks BA) is handed out.