Subdomain or not subdomain

As we develop a new site, we want to keep a lot of new informational pages on a sub domain. Our blog is on one and we get extra listings on google from it with every post- google will give 2 listings per sub domain, supposedly. Our new design firm discourages this claiming that this weakens the main domain. Our domain is pretty strong on its own organically and so I think folding out new informational pages- key word heavy and link heavy- would be a good move. Maybe they just don't want to do the work of carrying cookies across from domain to sub domain.
any opinions from the experts here?
Thanks for any help

Many times it create complications, so batter is to have a new domain by some more $$$.