In short, it is absurd to say, as some Bush administration officials do, that deficits don't matter, and it is equally fallacious to say, as some former Clinton administration officials do, that deficits are always poisonous and that we should strive to run budget surpluses at all times. Of course, by that logic, if a small surplus is good, then a bigger surplus is better. But there are threshold effects there, too. A small surplus may be beneficial, but a large surplus at the wrong time could be deadly.
Another area where people tend to ignore threshold effects is immigration. A certain amount is absolutely necessary to our economic health. There are many foreigners with skills Americans don't have, and we would all be poorer if we had no immigration at all.
Even illegal immigration is benign up to a point. There are jobs the native born won't do -- at least, not without being paid wages that would cripple many businesses and raise costs for things like fruits and vegetables that most people would find intolerable. Indeed, if it led people to consume less fruits and vegetables -- as significantly higher prices certainly would -- it would be bad for their health.
But once a certain threshold is passed, the cost of immigrants starts to rise above their benefits. In a worst-case scenario, they no longer assimilate and become a cancer within the body politic, the way Quebec is in Canada, where the francophone population is deeply alienated from the rest of the country. It would be very bad for the United States if the Spanish-speaking population were to develop in a similar way, isolated from the rest of society, but demanding special privileges and concessions from the English-speaking majority.
Thus the question of whether immigration is good or bad for the country depends crucially on its amount. Like salt, a certain amount is necessary, a little more is benign but too much can be cancerous, culturally and politically.
Keep the question of thresholds in mind whenever someone declares a policy to be absolutely good or absolutely bad. Whatever their position, such extreme statements are probably wrong.