There's more to it than that. The crank/rod system knows a lot of acceleration forces from the rotation and the oscillation of masses.
If you consider a 1 cylinder engine, you might be tempted to add a counterweight that has a mass equal to that of the piston and con rod assembly. This will even out inertia in both upper and lower dead centre, but as the weight swings through the horizontal plane, it pulls the engine to and fro. The part of the con rod that is considered rotating mass (about two thirds, centred in the big end) only goes so far to cancel out this horizontal force.
You will end up with a counter weight that is half of the resulting imbalance, so near 0,5 x (m_piston + 0,3 x m_rod). This will leave an imbalance force that is rotating at crank speed and has more or less equal value all the time. The engine vibrates a bit, but it doesn't jump or bump.
When you add a second cylinder you can logically give it a phase of 360 degrees. That will get you a nice even ignition interval (once every revolution of the crank) but leaves the issue of balancing untackled. In fact it gets twice as big, since there are now two identical imbalances working together. This is the classic Euro parallel twin bike from the 50's - 80's (Triumph, Norton, Laverda, etc). Bouncy bouncy.
So what if we place the cranks at 180 degrees? The ignition interval will be irregular, giving a bit less torque low down and idling that is less smooth. Oh dear.
The balance however is much better. The one piston going up will pretty much cancel out the one coming down. Only problem is that they are some distance apart, so the engine will want to rock left-right. To get rid of this effect you add a counter weight on the both outer webs (you want them wide apart, this gives more resulting couple with smaller weights). You end up with a rotating imbalance much like the single cylinder engine, but much smaller and the engine (rather: the crank) will try to make a movement like a diabolo.
Now we double up the 180 degree twin: two pairs of pistons/cranks that cancel each other nicely out but leave a rocking force (couple). By hooking them up we get a system that not only cancels out the forces at upper and bottom dead centre, but also cancels out the rocking resulting from the distance between the paired cylinders.
It has good end-to-end balance. Most pleasant effect: no more need for all that weight on the crank.
OK, so strictly speaking an inline four does not need counterweights at all!
Then why do factories put them on? The main reason they are there is that the crank does not turn just by itself, it get whacked by the force of the combustion every time. These pulses result in downward forces (logically) and in a piston going down faster that it did go up. But another blow on anther piston will see to that...
To dampen out these two sources of vibration we still need to add some mass to the crank.
Well... when you're out to get rid of all unneeded weight you may get those heavy counterweights off. Be prepared for an engine that scores very high in the NVH (noise vibration harshness) department!
In a V-engines things can be different. A racy Euro flat plane V8 may be considered as 2 4-pot engines sharing one crank. It even sounds like two. This engine can indeed run without counterweights. A regular US cross plane V8 does not have the inherent end-to-end balance, it must use counterweights on the webs. The reason they make these cranks is that it now acts as 4 pairs of V2's with each pair heavily balanced like the 100% balanced single above. Thing is: when the couterweight goes through 90' ('horizontal' in my single engine example), the second piston/rod on the same crank cancels out the force that would be pulling the engine to and fro. All this tilted 45' of course, it's the 90' between banks that matters.
This gives a very smooth engine, running at low speeds. Just what the American market wants from their family saloons. And everyone knows that burbling sound they make. It's the irregular firing interval that you hear.
Mind you: I'm not considering here things like secondary inertia forces due to the con rod being shorter than 'indefinite length' which makes the movement of the piston more like a sinus with the bottom half flattened.
In case you wondered (I did for years): no, the piston does not go down at the same speed as it goes up. Reason being that when going down the rod is tilting (big end goes sideways away from the cylinder centre line) adding to the downward acceleration. When nearing the bottom dead centre the rod gets vertical again, adding to the deceleration. When the crank goes through bottom dead centre and the piston starts to rise, the rod again tilts, taking some speed out of the upwards movement. Once the big end is past horizontal, the rod come straight, pushing the piston up faster that it would if only for the crank. You see the extra acceleration twice every stroke.
This is not a a very regular movement, and thus it has considerable forces (= vibrations) at twice the engine speed. The shorter the rod, the stronger this effect is.
Neither am I discussing engines with 3 or 6 cylinders that have better behaviour in this respect; you can imagine a straight 6 being a paired triple just like the straight 4 being two twins. It's the inherent primary and secondary crank balance of the 6 that makes these engines so smooooooth, helped by the overlapping of combustion strokes on anything with more than 4 pots. Of course this makes a V12 (that's two sixes!) nearly perfect. I want one.
And in boxers things are a bit different still. But that too would take us all night.
If you want to know more on this, do some Googling and/or Wiki on "engine balance" or "flat plane" or "cross plane". There are some very good and well illustrated pages out there.
Have a good weekend.