Agree with Mike about the mail (nice change) and also the left handed pose which is problematic.
The Roman Army and especially after the Marian Reforms fought as a highly cohesive unit in which everyone was effectively part of a larger machine as it were. Battle drills were strictly practiced and which involved predominantly formation tactics; most of which were shield focussed. The legions also operated a 'switch system' which involved those fighting in the front line rotating and falling to the rear by command (probably about every 90 seconds or so) as the next line stepped forward meaning that fresh troops were constantly placed in the front lines (even professional athletes like boxers do only 2 mins of intense fighting for a reason!) This system had to function like a swiss watch to keep the momentum going and was practised constantly in training to maintain efficiency.
Therefore, if someone had their gladius and shield in opposite hands to what was standard -i.e standard practice Gladius in right and Scutum (shield) in the left, that rotation technique could not work as men or equipment would come into unintentional contact as the formations changed. Put simply, those who were 'left handed' with a sword or anything else were effectively 'retrained' to combat their natural 'handedness' and change to the right. Many achieved it and one presumes that those who could not adapt, were simply dismissed from the legion. Unlike today's modern Western armies where they desperately fight to keep even poor recruits due to a shortage of volunteers, there were no such concerns in the 1st Century where service in the legions was seen as a good career for those of the agricultural and unskilled classes of society compared to the other options available to them. Rejection rates in this period were high amongst the legions unlike the later and more desperate times in the late 3rd Century onwards when the empire started to implode from invasion and incursions into their provinces.
Senior officers and of course gladiators etc. not operating in formation tactics could have been left handed but it is difficult to conceive of how a left handed legionary could have existed and performed their role (with weapon and shield reversed) in such a rigid and tactics driven army (legion) battle formation. Others may know more, but perhaps the contemporary images showing legionaries fighting only right handed is a good indication of standardised practices?
Oddly, the Gladius scabbard is shown here (correctly) being carried on the right side but the gladius is in his left hand meaning he would have to draw the sword across his body using the left hand which in turn, would then interfere or be obstructed by his shield (presumably then held in his right hand). In a compact line formation, this just could not work with interlocked shields which is why the Romans developed the 'reverse twist' action with the right wrist to draw the sword carried on the right and in doing so, negated the problem of not being impeded by the shield as they did so.
That said, doubtless someone may find a single solitary image and cite it as 'proof' to the contrary but this depiction of a left handed legionary is a strange choice indeed....unless of course the sculptor has used other historical sources to support this?
Gary