If block is removed from all lances, as someone already pointed out, lancers would be forced to resort to 1h/sheild for secondary/dismounted combat--or get another 1slot weapon of some kind. Polearm users and those players who use a 2h weapon as a backup (ie, cavalry who don't fight with a shield, minus HA), while not exactly fucked, would be nerfed considerably, being unable to block incoming attacks on horseback. This might not be an evil thing, however, and I'm sure those classes would still exist.
Playtesting would be necessary to determine the extent to which removing lance-blocking would negatively effect polearm and two-handed specialist cavalrymen. I suggest those who are reading this try playing without blocking, and without a shield, on horseback. I am simply curious.
As to the suggestion that jousting/greatlances be allowed to block: I do not exactly care, to be honest. The weapons are already at a severe disadvantage to other lances, being unable to thrust, and are next to useless on foot. Perhaps it would be good to buff them enough so that their users could at least defend themselves.