for any gaming build there is the important point of future proofing i.e being able to play games which will be released in say another couple of years. the link you provided shows an old game being played almost okayish at a low resolution and a low game settings. the point of that review is to point out to the fact that Intel is really focused on improving their IGP and bring it on par with AMD's APU IGP but whats noteworthy is that they still have a long way to go.
as far as future proofing goes, IGP wont let you enjoy most games especially those that employ technologies like physx. A discrete GPU adds/enhances gaming muscle by many folds and it is therefore very important to a gaming/rendering build.
here is a way to understand things. 24 frames/second is used to record videos and when frames are run at that speed, human eyen sees it as fluid motion. All these frames are however prerendered. i.e. all details of shadow, lighting and texture are already captured in every frame. An IGP will easily run any HD movie because of this fact.
when you game, it is a different story. playing an FPS for e.g., everytime you turn your head, the game populates your GPU with compute data to "compute" details of shadow, lighting and texture. remember, here nothing is pre-rendered and therefore every detail you see on the screen is being rendered in real time. To achieve the same fluid experience as with HD movies while rendering in real time, you should ideally be sitting at more than 30 fps.
The above requires a lot of parallel computing muscle which is only possible with discrete GPU's atleast for now.
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