I think the US Open would be great to have at Merion. However, after reading all about the 'high' qualifying scoring, and then watching on the weekend, I don't think the USGA would be happy with Merion's ability to 'defend par'.
Length would be a problem, if the course played as it did this week (which was pretty fast and firm, you have to accept that rain is a good possibility during an Open, as well)
The fact is, the US Open field has players that are much, much better than the US Am field. In fact, I was stunned at the relative short-ness of the 4 players on the weekend. Molinari, for example, tended to hit drivers in the 280 range.
Merion has nine par 4s under 440, and 5 par 4s under 350 currently. So you'd be looking at US Open that tested the ability to drive greens, and skip in checking 40 yard pitches. It might look a bit like St. Andrews from that perspective...
I still think it'd be a great championship if the US Open set it up like they did this week, and didn't worry about defending par.