This question may not look as an important one and in fact it is just out of curiosity.

But to the point:

Is Ethernet port blinking really useful?

Ethernet ports usually have two light indicators of activity (blinking leds). Yes, I know that they indeed indicate activity, but why do we need that for Ethernet ports while we don't for other interfaces? Is there any official explanation for that or is it just a matter of tradition nobody cares about? Or else?  

Looking forward to see your knowledge or ideas on the topic.

Yes, blinkenlights are your friends! The lights can be very helpful when diagnosing problems - especially when dealing with non-managed switches or remote diagnosis with unexperienced users.
Rule of thumb:

no link light = layer 1 (cable/port) problem
link light but no traffic = layer 2 problem (or higher) - VLANs, STP, port security, IP subnet mismatch, ...
traffic light constantly on but little useful traffic = bridge loop (or
constant collisions with a repeater hub), possible duplex mismatch

Many switches can signal multiple infos (sometimes using a toggle button): link speed (none/10M/100M/1G/10G), half/full duplex mode, slow/fast traffic, PoE available/applied, high error rate, inserted transceiver accepted/rejected and more. Most of these are very helpful when something doesn't work.
With other (shorter-range) interfaces, most often the other cable end is in the same room - that situation is rather easy to diagnose, even without link and traffic lights. With Ethernet, the far side can be in another room, another building, or even in another city. Twisted-pair cabling is mostly limited to 100 meter, but fiber can cover 100 kilometer or even more. You'll be glad to have a quick, local info light.
Note that the traffic blinking frequency is very often significantly reduced for the human eye - with a potential frame rate of more than 140,000 Hz just for 100M Ethernet, indicating each single frame wouldn't make sense. Well-designed devices attempt to signal various port load states by frequency (like constantly lit for 80-100%, fast blinking for 50-80%, slow for 5-50%, intermittent for 1-5%) but there's no standard sadly.
