That forward-thinking might be advantageous. If, say, Vietnam ever becomes a regional economic powerhouse, you have already locked in a trade agreement when you were in a strong negotiating position. This comes at a cost, however. Right now, and for the foreseeable future, Vietnam is not much of a market - so you are mostly granting import preferences to Vietnam. That's a slight win for consumers, as prices go down. It's also a loss for the current producers of those goods - some of them might live in your country. Now, on balance and in theory, it's a win. In reality, you just created a sub-group of your population who lost. They will be pretty angry, and probably a lot angrier than the general population is happier with slightly lower prices. Create a big enough trade deal and this sub-group can get really big - big enough to nominate and elect a populist.
But aside from that, free trade can really screw up a free market. If you are ideologically aligned with free markets, you can't ignore that free markets require free movement of goods, free movement of capital, and free movement of labor. The TPP largely focused on the first two. 2 out of 3 is not a free market, it is a distorted market that will not produce all of the usual benefits of a free market. They let the magic smoke out.