I believe that simple ideas can sometimes be presented in a simplistic fashion and in doing so the real meaning and idea can become lost or misinterpreted. I believe this is often the case with Krav Maga's concept of the simultaneous block and counter. This idea of a seemingly simultaneous defense and attack makes a lot of sense i.e. you will never survive a real-life aggressive assault by simply acting defensively; at some point you will need to attack/assault your aggressor - and if this is the case you might as well do it as soon as you can. The longer a street-fight lasts the greater the chances of sustaining a serious and/or life-threatening injury.
One of the issues faced though is that your attack has to be able to be effective. This is where many people have an unrealistic expectation of the effects of their punching/striking and their assailants reaction/responses to it. I often see instructors define an attackers response to a strike e.g. you punch the person in the face and their head goes backwards. Really? If a 300 lb guy is swinging in a committed haymaker whilst stepping forward and you as a 175 lb person try and block and at the same time throw a punch to the face I really don't think you can gauruntee the outcome. Different people respond differently to punching/pain etc. You can increase your chances of getting this result by making sure that your body dynamics are correct e.g. you are rooted, your strike involves your hips, shoulders and full body etc and that rather than punch you use an eye strike or cradle strike to the throat/neck or other soft target etc. But even then there is no gaurutee that you will create the movement you've been told to expect.
I'll often see people perform a standard 360 block, with a simultaeous punch where the body dynamics of both the block and the punch are questionable to say the least. They normally involve the body bending forward at the hips and the punch being thrown with the elbow out, so that the body is not behind the punch. The rush to getting the strike in, along with the block, means that the punch lacks and real power. Yes, in training it may produce a snap on the pads but that really doesn't mean it has stopping or major disrupting ability. Also with such equal and even effort shared between the block and the strike, there is often not enough strength and support given to the block e.g. it may not be strong enough to deal with the weight and power of a 300 lb guy's haymaker.
A more effective way to generate both power in the block and in the strike is to move both together by turning the body. Remember the 360 blocking system is based on the body's natural flinch reflex. This means the blocking arm will naturally move before the punch/strike starts. The idea is to piggy back the punch on to this movement. By turning the body towards the strike as you flinch you will move both hips forward. The hips are the powerhouse behind every strike and block you make. If you keep both elbows down, you will ensure that the forearm of the blocking arm is vertical i.e. defending the target and that the punch comes out from the body. When you block and strike like this there will be a short gap between the block connecting with the arm and the punch/strike connecting - if you clap your hands together twice, quickly you will get an idea of this timeframe.
There's another reason that you need the block to make contact first. The 360 defense is also used to defend against circular knife attacks. If the block and strike connect at exactly the same time, there is the danger that should your punch move the person back, they will end up dragging the knife across your blocking arm as they stumble back. By having the block clear the knife first you avoid this issue.
Rather than talk about simultaneous blocking and striking it is perhaps better to talk about an attack following a defense within the shortest possible timeframe. This also allows for control movements/attacks to follow a defense, where striking may not be appropriate e.g. it risks a person moving with the strike in a direction away from you, which may not be beneficial if you are trying to control them.
For me, the principal is really about always attacking and if I'm totally honest, pre-emptively, so a defense never has to even be made.