My uninformed opinion and personal concern for this goes something like this:

The technology and cost needed to build and maintain a system like this on such a massive scale would be really difficult to justify only for finding the occasional terrorist plot or for spying on American households. It doesn't make sense. Something like this needs to prove it's worth continuously, and would suffer from it's own success. In other words, if it was so successful that it was able to stop the handful of terrorist plots that have occurred since 9/11 and now there is only a rare new concern to follow, the system would become less relevant and increasingly hard to justify. Pumping countless billions of dollars into the system so that once or twice a year there is an exciting new lead for a terrorist plot in the making. I don't buy it.

Using the system to spy on the average American family also doesn't make sense. You would hear about a major surge in arrests from the system finding murderers, drug dealers, mob bosses, and collecting details of their crimes. It would be something to celebrate. Look at all of the good that this monstrosity is doing by stopping all of these bad people. There is no evidence of the NSA going after or caring about the little guys.

So we need a reason that provides continuous value and justification for the cost, maintenance, and controversy. Something like that can only be driven by the pursuit of money or power. If you can blanket collect data from every smart device in America, then you can collect reams of information about everyone who visits the US, including diplomats and foreign dignitaries. You can also create extensive detailed dossiers on every company executive, every financial institution, every state official, and map the relationships between powerful and influential people. You can create files of every personal wrongdoing for these powerful people, such as illicit or scandalous relationships, drug habits, or gambling habits, and create potential blackmail lists ready for exploitation when needed. You can follow the behind-the-scenes dealings for executives from major corporations to manipulate the global economy, or predict financial dealings.

You don't blanket collect information from every device in all of America to spy on American families or stop a couple of car bombs, you blanket collect information so that you can follow the powerful and influential rats as they run through the maze of our country. Nowhere to hide, no juicy secret kept private. That would give power to control corporations, governments, and global economies.

That is worth billions of dollars, and that will provide significant value every single day.