Was the cingulum militare any good as protection?

SHORT ANSWER

Experimental archaeology has led to the belief that what is known as the ‘apron’ (hanging leather straps with studs) probably did not have any practical defensive purpose and was actually a handicap “during violent movement”.

It has also been argued that straps would have been of limited use against both thrusts and slashes. Further, the 'apron' would have served no purpose against one of the most common enemies of the period, the Celts, due to their fighting style.

However, in the absence of primary source evidence to confirm this, it is impossible to be certain and some sources state that a defensive purpose is possible.

Finally, most sources point to the 'apron' as being both a fashion item and an indication of status.

DETAILED ANSWER

M.C. Bishop and J. C. Coulston, in Roman Military Equipment (2006), initially seem to sit on the fence a little:

It is thought that it offered protection to the area of the lower abdomen and groin, although practical experiments with reconstructions have shown that weighted straps swinging between the legs of running soldiers are more likely to pose an additional, unwanted, hazard in combat. A more attractive theory is that which see the ‘apron’ as a mark of status, reinforcing the soldier’s ego with its jingling components

Later, the authors seem more definite:

First to 2nd century ‘aprons’ served no significant protective function, and indeed could have been a liabilty to the running soldier.

In his article The Early Imperial Apron Bishop also says that

It has been traditional to explain the apron as an item of body armour designed to protect the soldier’s lower abdomen and private parts. However, this is to misunderstand not only the dynamic behaviour of a series of straps, but also to ignore the main traits in Roman armour development in the first century AD.

If a thrust from a sword, dagger, or spear were aimed at an apron, it would pass straight through, as the straps, with nothing to hold them together, would simply part. A cutting or chopping blow would most likely out clean through the straps, deflected from the studs (which never overlap and are not attached to each other) onto the leather.

On the trends in Roman armour design, Bishop notes

Many Roman foes were Celtic and fought with a downward slashing blow from a sword: this explains why Roman armour in the first century AD. is always reinforced on the shoulder area and why the helmets are designed to deflect downward blows. The whole philosophy of armour design is against the downward blow, not against the thrust to the midriff

On the left, "Grave stone of the legionary Caius Valerius Crispus, about 75 CE, here's [sic] he served in the Legio VIII Augusta. At his death he was 40 years old." On the right, "Detail of the full-figure tombstone of Annaius Daverzus of cohors IIII Delmatarum from Bingerbrück."

A more recent source is Stefanie Hoss’ The Roman Military Belt in Koefoed, H., M.-L. Nosch (eds), Wearing the Cloak. Dressing the Soldier in Roman Times (2011). She is more definite in her assessment:

This ‘apron’ has often been argued to have been used as a real or psychological protection of the wearer’s lower abdomen and groin area. Experimental archaeology has proven the contrary, namely that during violent movement (e.g. running), the weighted straps swinging between the wearer’s legs are apt to form a risk to the groin area. It seems more likely that the apron had no practical use, but has to be interpreted as a mark of status for the soldiers, maximising the characteristic jingling.

Adrian Goldsworthy, in The Complete Roman Army (2003) says much the same as Hoss on "the experince of modern re-enactors" and adds some details about the 'apron' which are consistent with the idea that it was primarily decorative. The 'apron'

might consist of one to nine straps, usually studded and with decorative metal terminals. Four to six such straps are most common. These may have served as a defensive function, even if the protection offered to a soldier's groin was more psychological than real.

Graham Sumner, in Roman Military Dress (2nd edition, 2009), also accepts the findings of re-enactors and notes the changing styles of the 'apron' during the 1st and 2nd centuries. He refers to the 'apron' as a "popular fashion". Further, Hoss notes that, in the 2nd century,

the introduction of enamel decorations... made the belt quite colourful.

Bishop, in The Early Imperial Apron, concurs:

It is easier to say what it was not for, but the unex— citing answer to the question must be that it was fashionable. Indeed, this is one of the most appealing explanations for the appearance of studs bearing portraits of the Flavian emperors.

Bishop concludes with:

Perhaps the best explanation of the apron lies in a sociological, rather than military, interpretation.

The 'apron', whatever its true purpose, was not around for long. It is first noted in the Flavian period (69 AD to 96 AD) but fell out of use well before the end of the 2nd century.

Whatever its purpose the apron gradually declined in size and does not appear to have lasted beyond the Hadrianic period (117 AD - 138 AD).

Hoss more or less concurs, stating it died out during the Antonine period (Antoninus Pius succeeded Hadrian in 138 AD).

Note: The term 'cingulum' does not appear to have been used until the 3rd century. Before then, the belt was referred to as a 'balteus'.