Clubs seem to think it cements the bond. It doesn't. A die hard fan will always be there.
This is the pertinent point here - but not quite in the way you're thinking of.
The club, perhaps contrary to what many (and I don't just mean here, I mean in general) believe, do realise that the die hards will always be there. The thing is, this letter wasn't written for the die hards. It was written for the people who joined because they were curious, or newbies to the sport, or did it for the heck of it, and so on, and are the people who may continue to associate with the team for the rest of their lives but are much more uncertain in their positions as continued match attendees. These fans are not your standard forum-browsing fair - they aren't the members of the SGs, they aren't the kind who berate the manager for not playing Poku, or think Kreis or Reyna should lose their jobs, or maybe even care whether the club plays in sky blue or forges its own identity. They are the ones who never supported the Red Bulls because they were never that "into" MLS until NYCFC landed at the door. Maybe they didn't even know the club existed up until the time when players were first being signed and season tickets were being sold.
These people are not necessarily the sports casuals of the world. I'm not saying they are people with no attachment to the sport - they are just the ones who do not view themselves as ultras, or die hards, or core fans or however you see the identity of the fans who are at the heart of everything.
These are the (paying) fans who the club can gain or lose. The club is writing this letter to show that it knows they are there and cares about them coming back. Now I'm sure we would all like the club management to organise a meeting and basically Q&A their way through the issues that we've had, but that's not necessarily what these middle-of-the-road fans want. The thing is, if they talk too negatively and make the letter a huge mea culpa, they may get more respect and a better working relationship with the likes of us, but it can actually majorly put off those who are still finding their identity with the team. If you start reeling off stories about how badly your team has done and how it's got to do better next year, they might go "oh wow, this team really cares about its public profile"...but they might equally go "I didn't realise this team was so badly-run that they feel the need to apologise", or worry that the club feels the need to drag them into levels of commitment that they don't want yet.
In short - these kinds of letters are not the right place to put apologies, there are better avenues for that. That's just plain sensible marketing. Treat the die hards like die hards and treat the more laid-back fans like laid-back fans. Don't treat laid-back fans like die hards or you start alienating people and make it hard for people who don't want to become ultras to join the club.