CGA vs GGSA - why unsealed spellbook hecarim?

The answer should be obvious. If you go and unsealed spellbook, you can have ghost up for pretty much every fight. Ghost is very important to hecarim as it gives both mobility AND damage. An additional bonus is that others will probably not be as likely to know the exact time when you have smite available again after using it. So say, you smite drag and then rush over to contest baron - most junglers wouldn't have smite available again necessarily while you would due to the much lower cd.