One may ask whether the outer arch is sharpened into a blade at all... Do we ever see it cutting anything? We see that part of the weapon being used for pushing the opponent away on several occasions, and there are no cuts in evidence as the result. The modern bat'leth might simply be a warhammer with four pointed ends for piercing the enemy armor, and the front curve would be but a dull shield for blocking enemy attacks by catching his blade.
Timo Saloniemi
I like your overall take on this,
Timo. And in the above paragraph, I think there's a great validity to this idea overall; for the most part you only see the inner arc of the blade being used for parrying and torquing opposing blades - and you never see any Klingon sharpening this section that no doubt would continually absorb damage.
However there is a scene - I believe it is when Worf is demonstrating the use of the bat'leth to Alexander, but I can't quite place it in memory - when he wields the blade using his Tai-Chi-esque Klingon fighting art,
Mok'bara. Worf being Worf, and a cut above your average Qo'noS-born warrior, is the only one we see to wield the bat'leth flowing through numerous kata-style stances.
Most other Klingons swing the blade, flick it or brute force it. Worf's forms, though, involve cutting arcs that very much do imply using the inner blade to slice. Though we never see anyone actually do that, we do see plenty of missed bat'leth swings throughout the franchise, even Tom Paris and Quark dodged a few.
For the punishment it's fated to take as a parrying weapon, though, I would not want to spend my energy repairing any sharp inner edge. Besides - you can do plenty of damage with a dull piece of wood, let alone a meter-long dull steel edge whipping at you.
Verdict: I dunno if it's sharp or not. The bat'leth, as much as a fighting weapon, is also a parrier to test the brute force of friends and enemies alike. The point of Klingon combat is not to kill and die - it is to honor yourself in battle. To this end, they don't always take the shortest path to killing. They
want to parry and test each other's power. See the headbutt test of Data as a measure of his mettle.
Hence the constant clashing. (Well, that and weekend warrior actor training). (Don't get me started).
So yeah, a dull blade makes perfect sense, but - so would a sharp one, if it were previously a farming implement. So...either way?