There are just five operations where the usual multiple master model breaks down, and the Active Directory task must only be carried out on one Domain Controller. FSMO roles:
PDC Emulator - Most famous for backwards compatibility with NT 4.0 BDC's. However, there are two other FSMO roles which operate even in Windows 2003 Native Domains, synchronizing the W32Time service and creating group policies. I admit that it is confusing that these two jobs have little to do with PDCs and BDCs.
RID Master - Each object must have a globally unique number (GUID). The RID master makes sure each domain controller issues unique numbers when you create objects such as users or computers. For example DC one is given RIDs 1-4999 and DC two is given RIDs 5000 - 9999.
Infrastructure Master - Responsible for checking objects in other other domains. Universal group membership is the most important example. To me, it seems as though the operating system is paranoid that, a) You are a member of a Universal Group in another domain and b) that group has been assigned Deny permissions. So if the Infrastructure master could not check your Universal Groups there could be a security breach.
Domain Naming Master - Ensures that each child domain has a unique name. How often do child domains get added to the forest? Not very often I suggest, so the fact that this is a FSMO does not impact on normal domain activity. My point is it's worth the price to confine joining and leaving the domain operations to one machine, and save the tiny risk of getting duplicate names or orphaned domains.
Schema Master - Operations that involve expanding user properties e.g. Exchange 2003 / forestprep which adds mailbox properties to users. Rather like the Domain naming master, changing the schema is a rare event. However if you have a team of Schema Administrators all experimenting with object properties, you would not want there to be a mistake which crippled your forest. So its a case of Microsoft know best, the Schema Master should be a Single Master Operation and thus a FSMO role.