AD LDS vs AD DS

However - when you install Exchange, does it not require that you extend the schema in the actual AD DS server role? Exchange may have certain features that make use of AD LDS, but I'm wondering why you can't, or at least should not use AD LDS for any and every application that would otherwise extend the schema?

My textbook (70-649) reads "Adding to the schema for an application such as Exchange Server is appropriate because it provides a core networking service. AD LDS is not the correct choice in this case." Can you maybe elaborate on that reasoning for me? that seems rather weak.

Thanks