I think the idea is to give people more options, and to ensure that realms only keep the treaties that they really need--to prevent the eternal alliances that you see between certain realms. Or at least, limiting the number of alliances that can be easily maintained.
Where are these eternal alliances you speak of?
I see war on all continents that I know of. Long-lasting alliances already have great strains on them: player boredom, short memory, and noble replacement. You might be best buds with the realm you just helped form, but if you just sit on your ass without talking to them for the next year, no treaty will bring back that lost relation.
