Just using the Premier League as a point of reference, the bottom three teams probably don't vary that much in quality from one season to the next. 38 points is a fairly reliable point total for ensuring survival (over the past five seasons, the team who finished 17th has averaged 37.4 points).

It's been well-covered that the relegation threat does make for a more exciting end of the season for teams that would otherwise have nothing to play for. We've all been to late-season MLB or NBA games between two bad/mediocre teams. It's an exercise in pointlessness. MLS gets around this somewhat by putting over half of its teams in the playoffs, so there are still important implications halfway down the table. Which way is better is a matter of preference (I prefer relegation to a bloated playoff bracket).

I think the most interesting point Klinsmann was making in calling up Miguel Ibarra from Minnesota and Jordan Morris from Stanford was that it's difficult for him or whoever's in charge of the national team to evaluate domestic-based players when everyone is not on a single spectrum. The only time NASL, USL, and MLS come together is in the Open Cup, which a lot of MLS teams don't give a shit about. In truth, there's probably very little difference between the bottom end of an MLS roster and the top players in the NASL. Putting the pro leagues together would at least give us some sort of common continuum.

Here's why promotion/relegation is not going to happen: The owners won't vote for it, and MLS would have to cede the player contracts to the clubs. Obviously the owner part is self-explanatory, as they want to protect their investments. The tough one is that the league has no interest in letting go of those contracts; they still feel the need to keep the teams as balanced as possible. The problem is that the constant expansion (soon to be over-expansion at the rate they're going) keeps stretching an already-thin domestic talent pool. So, unlike comparably talented leagues like Holland, Portugal, and Russia where you have a few stacked teams at the top and a bunch of chaff below it, we'll have 24 mildly competent clubs, with a bland, homogenized overall product.