Is there any advantage for using a spear instead of a glaive? From the outside looking in, a glaive can do everything a spear can, is harder for an enemy to grab, can be used as a slashing weapon, and can break shields.

Why would anyone use a spear instead?

Because spears have no weight restrictions.

Just to add what Dangus said:

1) They're generally lighter so they tend to be faster than glaives.

2) They're easier to maintain because you only have the head, haft padding, and pommel to worry about.

3) They don't require as much material, i.e., foam, tape, glue, etc., so they tend to be cheaper to build.

4) If you can find the right cores such as bamboo or carbon fiber, you can build them longer than 8' giving you significantly longer reach. Building a glaive on bamboo is generally not a good idea since the bamboo is far more likely to crack when swung.

5) You can build them with a lower profile which allows you to hit smaller targets.

This^

Getting good at spear lays the foundation for a very good glaive user.

As someone who just picked up a glaive after using a spear for the last few years what everyone else said is absolutely correct. You are right that the glaive is more versatile, but the spear can shoot of smaller targets. Also the extra weight of a glaive is something that a lot of folks can't use all day, they even start to drop stabs after a while. I know it isn't that much weight, but when you are fighting for 6-8 hours in a day it gets you over time.

Thanks for the input. I'm glad to hear that a spear is a legitimate tool and not just a Diet Glaive. It also sounds like a good polearm to start with.