It's absolutely true that Travis Mayweather was the least utilized of any series regular in a Star Trek since the original series, where Sulu and Uhura especially were used mostly as window dressing, and Chekov mostly for Russian anecdotes. It got to a point where fans joked that he could be played by a cardboard cutout with equal effectiveness to actor Anthony Montgomery.
This became true mostly in the last two seasons. As point of fact, he got a spotlight episode in each of the first two seasons. Yes, faint praise to a certain extent. As I understand it, half the character's potential was pulled out from under him when it was determined his own backstory worked against the concept of the series. Mayweather was a Boomer, part of the human space experience that lived on starships before Starfleet made it routine. Of course, Enterprise was all about the formative development of Starfleet, pushing the boundaries of human space experience.
In short, being a Boomer meant becoming obsolete. Mayweather joined Starfleet to be a part of this development. His old colleagues and family didn't uniformly see that as a good thing. "Horizon" is all about this conflict, brought to a point when news reaches Mayweather that his father has died. Like the rest of the latter half of the second season, the episode serves as a status marker, showing how far things have developed, this time from the perspective of the pilot who was the most natural spaceman in Archer's crew.
I always wished the character would receive more material. I'll admit to being fascinated by the potential of the Boomers to offer a commentary on Starfleet. Unlike the show's creators, I still thought there was a chance that Mayweather's Boomers roots could provide insight into developments well past the second season. I assume this didn't happen mostly because the writers found other things more interesting to them.
"Horizon" doesn't make Mayweather out to be a hidden treasure of the series, but at least an underappreciated one. I would argue that there was a lot of worth to having a recognizable character lace his way through the series, regardless of how much material he got. He was still more important and distinguished than Sulu or Uhura. Other incarnations made use of their casts to varying degrees of effectiveness. Mayweather rates below all of them, but he still leaves an impression. An episode like "Horizon" is like a reward for noticing him at all. You're a true fan of the series if you appreciate this one.
franchise * series * essential * character
Memory Alpha summary.