Very few realms have any domains that monopolize a specific aspect of life (temple, guild, source). There are a few exceptions of course, the OIT has a virtual monopoly on temples in Diemed for example. Some mages also enjoy a monopoly - or near.monopoly, but that is mostly because they are so rare.
The reason is that having a monopoly gives you a whole lot of power, and as a sovereign you don't want that. As long as you rule a LG realm with 3 LG domains played by your very best friends you're OK, but as soon as that's not the case you're in trouble. If one, or god forbid 2 or 3, band together against you you're in a whole lot of trouble! Sure, you could crush that guild eventually, but what will it cost in lost revenue and wasted actions?
What you DO want is one powerful temple that is loyal to your rule, one powerful guild that is loyal to your rule and one powerful wizard that is loyal to your rule. Powerful, but not with a monopoly. If they have much more than 1/2 available holdings you should be thinking things through, and by the time they pass 2/3 it might be too late.
So a wise ruler tries to plan long-term and balances several domains of the same type against each other. You don't even have to have the most powerful domain of a certain type as your state faith/whatever. You could help balance the power of a big temple by making a smaller temple your state faith.
Of course it doesn't always work out this way. Politics is a strange beast, and sometimes things end up very differently...but this is what a good ruler should strive for.