I have an ongoing difficulty with the widespread preference of some wargamers for depicting those who fought at Waterloo in greatcoats. There is this persistent notion that from 11:20 am or thereabouts on 18 June 1815, men would have marched and fought in heavy woolen greatcoats because it rained the night before. Together with this idea is that men went into battle with shako covers. This prevailing view and desire has in turn steered sculptors and manufacturers to provide such figures for 1815 - they'd be mad not to of course.
I do not dispute that the weather in Belgium can get cool even in high summer and that weather can change with radical dips and rises in temperature. The morning would have been misty as it was in late May 2012 when I last visited - it was cool also on my visit. Having said that, there was no rain on the day in question and it had ceased by 7:00 am at the latest by all accounts. The meteorological study by Wheeler and Demaree demonstrates the previous evening's downpour and electrical activity was nevertheless a shallow trough of low pressure following a scorching previous day.
I suggest that it would have been and still is wholly impractical to attempt to march and fight with a sodden greatcoat by midday when the mists were clearing and the weather warming. If worn the night before (and I certainly would have if I'd had one) it's a matter of drying it as best one can and then rolling it and strapping onto the back-pack or saddle. Once on the move, greatcoats would have weighed the men down significantly - heavy on the arms, cumbersome and increasing fatigue for the French in particular who did most of the foot-slogging over muddy ground. An active soldier will have generated his own heat and combined with rising temperatures (even if only a few degrees) in what must have been developing as a humid zone would have been galling. Worse that that, it would simply have been insane.
An active person in the great outdoors is going to dry quicker under even a feeble sun with less on - not more. The uniform jackets were all wool also and should have provided sufficient insulation against any breeze which may have prevailed and if sodden, would have been heavy enough on their own without the assitance of another and thicker layer of soaked wool on top of it.
Of course, there's no such thing as uniform when it comes to uniform. There are references to members of the French Old Guard wearing greatcoats but more to cover civilian attire when a soldier was without a full uniform. For the appearance of the Guard, appearances were almost everything in front of their Emperor. I regard this; however, as an exception to the general rule. I say leave your greatcoated infantry for the Russian winter campaign but keep them away from Waterloo.
On the subject of shako covers - they are an item for campaign as opposed to the day of battle. They were no more likely (on either side) to retain their shako covers by midday than I would carry an open umbrella for four hours after it had stopped raining. As with all items of uniform, given sufficient preparation time (and there was plenty on the day of battle at Waterloo) regiments would have gone into battle as splendidly arrayed as was practicable to identify themselves (particularly important amongst the Allies), increase their own morale, to impress or even intimidate the enemy and display before their own command. It was a different era to the modern.
My difficulty in the persistence of greatcoats and covers only truly extends to when I cannot purchase figures sculpted without them - which is the case for my Elite Miniatures Nassau regiments. In any event, this is my considered opinion - very much considered but nevertheless very much an opinion only. Please feel free to disagree.