I am not makeing my character less effective, I am making him diffrent.
With expertise instead of (some other feat), you can make that argument, because the other feat might truly be more effective, but the kukri argument doesn't work.
Let's say you're choosing between using a rapier and a kukri.
Rapier: +3 Proficiency, 1d8 damage
Kukrui: costs a feat, +2 Proficiency, 1d6 brutal 1 damage
So, the rapier is 1 less feat, 1 more attack, and .5 more damage. That's factually less effective. If you want to compare using an off-hand weapon to using a light shield, sure, there's an argument to be made there, but pure raw weapon comparison, system mastery says it's a mistake.
Me, I'd rather the game didn't actually work that way, but there you go.
ok so lets take weapon users...
I start with a 16 stat, have a +2 prof, and no epic destiny benfirts to attack or attribute
you start with a 20 stat, have a +3 prof epic sestiny +2 stat, and a class +1 to hit
+5 diffrence to hit
with you having expertise it is up to +8 at epic +7 at paragon and +6 at heroic
Sure, for comparison let's say we're both strikers and do 1d12 damage base (greataxe vs. fullblade, say) with a d8 extra damage kicker for striker goodness, bumping every die every tier for encounter powers and scaling, weapons going to +3 at 11 and +5 at 21st)
"You":
1st - 1d12+1d8+3 at +5 vs AC 15 (14 avg damage at 55% hit chance, 7.7dpr)
11th - 2d12+2d8+7 at +14 vs. AC 25 (29 avg damage at 50% hit chance, 14.5dpr)
21st - 3d12+3d8+11 at +23 vs. AC 35 (44 avg damage at 45% hit chance, 19.8dpr)
(3,4,6 stat / 0, 3, 5 enh / 0 feat / 0 class / 2 prof)
"Me":
1st - 1d12+1d8+5 at +10 vs AC 15 (14 avg, 80% hit chance, 11.2dpr)
11th - 2d12+2d8+9 at +20 vs AC 25 (29 avg, 80% hit chance, 23.2dpr)
21st - 3d12+3d8+13 at +31 vs AC 35 (44 avg, 85% hit chance, 37.4dpr)
(5, 6, 9 stat / 0, 3, 5 enh / 1, 2, 3 feat / 1 class / 3 prof)
Which results in about a 50% difference early on, widening to almost 100% difference later on.
And that's before adding in other damage bonuses, like feat or item. Every damage bonus you add stretches the gap just a bit more. The real craziness starts when you assume the optimizer also starts picking up extra attacks, while the non-optimizer doesn't... but that's separate from expertise. The basic flaw that "2 type As do the job of 3 type Bs" isn't really a game benefit. Choosing a 16 vs 18 vs 20 is a real choice - you can use those points somewhere else, you can get particular racial benefits, etc. Choosing a +2 vs +3 proficiency is usually a real choice - damage vs accuracy.
Choosing expertise vs not... rapidly becomes a trap choice. Hence, why we get threads like this, and why some of the WotC developers give out expertise for free in their home games, and others added extra benefits to the feats to ensure that people would take them without as much complaint.