well, spears would still be faster without a shield (because you could also use it as a staff), but yea, you can use a shield with em.
Just saying that some pole-arms couldn't be used with a shield, while most can be either 1H or 2H. Meanwhile pure staff-combat weapons wouldn't have a spearpoint, so couldn't use a shield.
Although ... a Lance pretty much just has one use with or without a shield .... which is a couched attack using momentum.
I mean, have you tried to NOT use a horse and still attack with a lance? Your pathetically slow, and the only think you "might" accomplish is to de-horse an opponent ... yet good luck with that, you'd have to cripple the horse somehow or stab its neck.
But yea, its impossible to parry with a lance ... and its incredibly slow (its only attack is momentum based), and you can ONLY attack an opponent from 10 feet away. If they get any closer your dead.
Conclusion?? Lances should be only usable while riding a horse. Or maybe they have a base 2 attack and get +10 attack on horseback, and +30 attack on a charge (moving X tiles in one direction, while on a Mount)