Here are the pros of adding cooldowns:
1) They add a layer to gameplay - you now have options of what to cast based on timing. You can create rotations to maximise damage.
2) They add a layer to itemisation - you now have one more stat to like on gear, and can create interesting builds based on CDR.
I'd like to hear the cons! You've mentioned they're bad, twice, without saying why, actually.
And finally, actually, the way the current lance is working -- it's a lance (you move while hitting monsters with it). Spears would be more of a jabbing affair. Again, it's interesting to hear why you want this change in particular!