The Dangers of Government by Executive
The more a president governs unilaterally, the greater the stakes of every election, and that's a bad thing.
There are lots of things one could say about Trump's invocation of an emergency statute to "build the wall." Other commentators, including Ilya, have said most of them, so I'll refrain, but in short it's a terrible idea, and I hope the courts stop it.
I did want to add one additional consideration that goes beyond the issue of the wall and goes to the general issue of presidents acting unilaterally on significant, controversial issues, regardless of whether they have the technical legal authority under broad, vague, statutes.
During the Obama administration, defenders of presidential unilateralism argued vociferously that (a) the president was elected to get things accomplished; (b) Congress, via Republican majorities in the House, and later the House and Senate, was being obstructionist; and (c) therefore, the president was within his rights to use his full authority to govern unilaterally, even in the face of longstanding contrary norms. For example, Obama, like Trump, was stymied by Congress on his preferred immigration policy, so he used his broad statutory authority under the immigration laws to resolve the "Dreamer" issue indefinitely, using that authority far more broadly, more consequentially, and in more direct defiance of Congress than any president had previously.
As a constitutional matter, I think this argument and analogous arguments made (less coherently) by Trump administration defenders have things backwards. Congress and not the president is given the legislative power, so outside of certain military and foreign affairs matters it's Congress, not the president, that is elected to get things accomplished. So in any showdown between the president and Congress, if one party can be deemed obstructionist it's presumptively the president.
But my objections to presidential unilateralism go beyond the constitutional. In my view, historically one of the great advantages of living in the United States is that most people did not care about national politics nearly as much as in other countries. This had the advantages of not wasting people's time thinking about politics, allowing people of differing ideologies to get along, and just in general making life more pleasant by limiting the practical importance of elections.
Of course, a major reason people did not care about national politics that much, even if they had strong feelings about particular issues, is that the national government historically had relatively limited powers. But another reason, one that transcended the scope of federal power, was that it our Constitution makes radical change, or really any significant change, very difficult. You need to get a piece of legislation through each house of Congress (each of which has various internal rules that make it hard to do so) and then get the president to agree.
This obviously has its downsides, as it creates a huge status quo bias, and the status quo is often far from ideal. On the other hand, voters could be assured that whomever got elected, Republican or Democrat, major changes were unlikely. This was especially true because until recently, the parties were divided more on geography and the cultural background of their voters than on ideology.
Over the last forty years or so, however, the parties have become strongly divided ideologically, which by itself raises the stakes of national elections; electing a Republican will bring into government a very different group ideologically than electing a Democrat. Moreover, delegation of authority by Congress to agencies gives the president a fair amount of discretion to change policy through his appointments of executive officials.
Nevertheless, the stakes remain relatively limited so long as the president adheres to traditional norms and refrains from major policy innovations without going through Congress. Once the president instead chooses to in effect "legislate" on important matters unilaterally, the stakes get raised substantially, and the U.S. becomes more like parliamentary systems where every election seems like life or death to partisans, inevitably increasing societal tension over ideological and other differences.
So, constitutional structure aside, why is presidential unilateralism bad? Because our system was designed to make major shifts in government policy difficult, and that's a good thing because it lowers the stakes of politics. We had experience in 1861 with what happens when a significant part of the country believes that the national government has become arrayed against it, and it's not an experience we should want to repeat on any scale.