Economist chart gives you some sense of why Taiwan's leadership, if not its people, was so eager to sign the Economic Cooperation Framework Agreement. Taiwan's exports top its imports by roughly 4-to -1.
Yes, it's always been the case that Taiwan had a trade surplus with the mainland, but the explosive growth in exports over the past decade changes things quite a bit.
First, Taiwan came to realize how big the China opportunity is for its export-driven growth as an island economy. Second, Taiwan needed to move to protect that slice as China enters into free-trade agreements with others in Asia.
Taiwan simply could not be left behind.