"special" ID card v "normal" Belgian ID

The only advantage is that the card is obtained by the employer and the staff member receives it in internal mail. So no need to queue up at the commune to register. Ditto for renewals, all done by paper applications.
Otherwise it's rather a hindrance than an advantage. There is no electronic version available, so many of the services available to holders of normal Belgian ID cards are not available to the holders of special identity cards (such as checking online whether your lease has been registered).

Oh, and another hindrance: unlike a normal Belgian ID card, the special ID card can't be used as a travel document in EU countries.

but they do mean you pay very little tax.

Tax!? They don't know what he hell tax is! They whinge about tiny contributions that are frequently exceeded by allowances! Unless we are talking about making sure member States charge huge amounts of tax and pass it on to the commission of course. They are good at that.

I await yet another one to tell us how all is good because hey, they did a "competition" and so deserve high salaries and special rates of tax that no other EU citizen could dream of.

Politicians have always managed to enrich themselves at public expense, but the Brussels mob have managed to take it to a whole new level.

One more advantage: unlike with a normal Belgian ID, with the special ID card it's not compulsory for the card to display the address of the holder, which means you don't have to get a new card every time you move. A few more hindrances as well: you cannot apply for Belgian citizenship if you have the special ID card, and you are not entitled to unemployment benefit while you have the card.
So if you have the option to choose and you are thinking of whether you should get one or go with the normal Belgian ID: if you are a Contract Agent it's more useful to have the normal one; if you are a permanent official it probably is more convenient to go with the special ID card.

You won't find many applying for Belgian citizenship because they'd then lose their Expat allowance!

Shhhh. You'll soon be accused of jealousy and be reminded that there is a "competition".

I'm sure it will irritate Mike no end to know that the majority of civil servants, including European ones earn less than they would if they were working in the private sector, comparing earnings for like-for-like responsibilities, qualifications and experience.

If you are working in the EU institutions, there are people there who will give you much better advice on the ins & outs of the "special" ID than anyone here will.