Back when Blitzkrieg first arrived, I too quickly dismissed these guns, but after a few games with them, my opinion has been changing. Although still inadequate against many targets, they can go a long way towards protecting an intermixed infantry platoon, which as light guns they can now in v3 accompany at full pace.
Flames of War represents these Early War anti-tank guns similarly. The Pak 36 is pretty typical with 24" range, RoF 3, AT 6 and FP 4+. The Polish 37mm is statistically identical. The British 6 pdr is a bit better with AT 7. The French 25mm is a bit worse, with a range of 16" and FP 5+. They are all cheap, with the average price being around 100 points for 3. They are also relatively plentiful, being widely available in most infantry force organizations.
Now, when Blitzkreig was released, in the dying days of v2, I pretty much ignored these guns. I had LW on my mind, where I was never much impressed by AT guns, and new to EW, and wanted to spend my money on sexier purchases like tanks or infantry. In retrospect, this was a mistake. I did build a German (and a French) infantry company, but with 2011 Nationals looming I switched my energy to building my chosen tank force full of lots of cute and cheap tanks.
I have since returned to the Europe of 1939 and 1940, and in so doing have begun to reevaluate the Pak 36 and its cousin guns more favorably. At NOVA, I played a game of Poles vs Soviets. The Polish force had a force of three anti-tank guns that decimated a Soviet light tank company. Then, I finally fielded some PaK 36s in a German vs French game with Steve. They performed admirably, even if they were eventually overwhelmed. I am now working on a Polish infantry force, for which guns of some sort are more or less mandatory, given the paucity of effective Polish tank options. So I'd best learn how to use them.
Let us consider the virtues of anti-tank guns (mostly the PaK 36, but my comments can be applied to others, too.) First, the PaK 36 has pretty good stats. AT 6, 4+ FP is comparable to the main cannon of a 38(t) or a Panzer III. (For comparison, the performance of AT 6 vs FA 3 is equivalent to AT 9 vs FA 6. So if you can't do the math in your head on the fly, imagine you're shooting a Panzer IIIM against a Sherman in MW.) 24" gives it a range as good as any Blitzkrieg tank and better than 16" pop-gun boxes like the Panzer II. Second, the PaK 36 is a light gun, which means it can (under v3) move at the same speed as infantry, and it has a gun-shield which means it will have bulletproof cover without digging in.
The disadvantages of light guns are obvious. They can get bogged in difficult going. They have a crappy 5+ save if they have moved and/or shot. If you shoot them from the rear, their gun-shield is worthless. Plus, they come in separate units, so an enemy can fire at them and ignore any nearby infantry.
Ok. So, what good are they?
In my experience, a well-handled unit of vehicles will almost always defeat any anti-tank guns. The tanker can use the superior mobility of their forces to concentrate fire on the guns, while splitting, reducing or avoiding fire in return. Ideally, the tank commander will engage the guns just at the tippy-edge of range, where all the tanks can concentrate on a single gun and destroy it in one move, before it can return fire. Even if this is impossible, a good unit of tanks can stack up behind cover and avoid the worst effects of a return salvo. Thereafter the light guns become extremely vulnerable as their save drops to a measly 5+.
I figure the purpose of anti-tank guns like the PaK 36 is to slow down enemy tank platoons rather than destroy them outright. As long as the guns live, they inhibit enemy armor from assaulting my infantry. Few enemies want to take an AT6 shot in the side as they close. In many cases, they will not want to enter range of my guns at all, or if they do, to pick away at my guns one at a time. If all my stuff is Concealed and Gone-to-Ground, then my opponent must get within 16" to attack me. And at that range, their light tanks become vulnerable to my guns. Each turn spent isolating and destroying my gun teams is a turn in which they do not make an assault on my infantry. Of course, if I can actually destroy enemy armor, too, that's gravy!
The virtue of the PaK 36s comes when I intermix them with a large infantry platoon and (hopefully) some HMGs. Most Blitzkreig infantry lists have multiple slots for anti-tank gun teams. With careful list design, I can create one or more pairs of infantry and guns. With v3, my whole combined arms death blob can even move together at the same rate, with all its elements protecting the others, and still be slow to break.
The turns I gain will (hopefully) facilitate my victory. In many scenarios, a defending army wins after a certain number of turns, or a certain amount of time. In others, holding on one part of the board will free me to move my armor on another, pressing for a victory. Even if I can't win just by surviving, and I have no armor of my own, I can hope that my guns inflict sufficient attrition that when the enemy does assault my infantry, their reduced tank platoons cannot defeat my remaining infantry.