This is not really relevant to whether the MLS and LigaMX should merge, but 5 Welsh teams play in the English system. Cardiff is in the PL, Swansea in Championship, Newport County is in L2 and 2 other teams are in English feeder leagues. There also are some English teams in the Welsh leagues I believe. And I think the Welsh leagues are fairly new as things go in UK football (1990s? maybe). Traditionally I think they all played in English leagues and so the movement was the other direction.That's the same as Cardiff being in the english league system while the other Welsh teams aren't. Would you want to see all the Welsh teams added to the system post-facto?
The main reason for me is down to the fact that I don't want to have to wait 4 years to play against team X. I doubt they're going to do a pro/rel system, which means it'll be just one big league.
But like I said, I don't see how this affects one's opinion of a Mexico/US soccer league merger. I'm probably mildly on your side against it, but I need to know more before committing.