(If you do not care about realism, skip to the next paragraph)
Since the dawn of makind, man has always thought that attaching a long pole by velcro to you back was ridiculous. Throughout history, due to Native limitations, huge weapons could either be attached to the back or made to fold and fit in the pocket. This has changed - the brilliant cRPG modders introduced the "unsheathable" flag. It can be done! we have the technology! So now, large medieval weapons can be treated as in real life - carried in hand and dropped to the ground when another weapons was to be used. There is no reason we should not apply this to most weapons in game to simulate the real limitations vs. advantages of large 2H/pole weapons.
Gameplay balance:
There is continual discontent with the number of people using powerful big weapons. The weapons themselves are fine, but when sheathable, they bypass the main weapon disadvantages - a poleaxe carrier can still use a shield and near insta-switch to his big whopper when he enters melee. He can have it on his back when lancing and then pull it out when de-horsed. etc. I believe that such weapons should not be used by cavalry (even ignoring the history aspect) or in combination with any ranged weapon. Infantry that choose to wield them should suffer the attached drawbacks (no shield switching, no switching weapons back and forth). On top of that, some sheathable weapons have a very similar counterpart which is for some reason unsheathable - so either both should be sheath or unsheathable (preferably the latter).
Benefits of "cannot sheath" flag:
As a compensation (game play) and appropriately (realism), the slot requirement of these weapons should be reduced - they are carried in hand after all. I think 1 or even 0 slot is acceptable in these cases. It will not help much to any ranged character because these weapons will drop to the ground the moment the player tries to switch to his ranged weapon.
Weapons that really should be unsheathable:
--------------------------------------------------------------
* Elegant poleaxe
* German poleaxe
* Great long bardiche + Long bardiche - the only difference of the "short" version is the tip of the blade.
* Glaive - The Swiss is 2 cm longer, lost the swing and cannot sheath. Glaive can keep the swing, but not the sheath.
* Poleaxe - I am sorry, but it looks idiotic sticking half a meter above the carrier's head.
* Long hafted blade
* Awlpike - 1 cm less than the Ashwood pike makes it sheathable?
* Long Maul
* Long hafted spiked/knobbed mace
About the long axes:
Axes can and were carried on back. You can find scabbards for wood axes in stores - but these were much shorter than the long axes. Again, the main argument here is the game balancing, excluding these weapons from becoming sidearms:
* Great long axe
* Long war axe
I would go on and on with the unsheathable polearms list, but for game's sake and the outcry that will come, we can leave a few sheathable polearms.