I know its a pain isn't it? i liked them in Moo 1&2 where they were a permenent thing untill you either break them off or do something to make the other race cancel it.

They are probably better balanced in terms of gameplay, but it is a pain, because you really have to make sure you have the liquid cash on hand in order to make the trade deals.

I will usually even pay some or most of other races part of the deal, because it's worth it in the long run, and it's a really good way to make nice diplomatically.

I actually like them better as a game mechanic this way. You are investing a significant sum of cash up front in order to recoup more over time - but not forever. The fact that you're willing to do this makes the dippy bump acceptable - sneakiness aside, you are essentially saying "I not planning on attacking you anytime soon". Ditto for research agreements.

I use the 20 turn expiration as a reminder to check for tech trades. :)