If I were writing the letter, I'd probably concentrate more on the inconvenience the missed connection caused (assuming it was more that just losing out on some time at the destination). Nothing wrong with asking for some compensation - no ask, no get.

While mechanical problems are the responsibility of the airline, they're also largely unavoidable - no airline has figured out how to completely eliminate them, and undoubtedly never will. With the cost of idle airplanes, no airline has any truly "spare" aircraft sitting around, especially at non-hub airports. Anytime a "spare" aircraft is pulled out, it most likely was pulled off another route.

Unfortunately, in what appears to be a routing of AAA-HUB-BBB, if the AAA-HUB flight experiences delays there's the distinct possibility that passengers will miss their HUB-BBB flight. The alternative, from the airline's perspective, is to delay the HUB-BBB flight enough that connections are made but that then may have a snowball effect and everyone on the next flight (BBB back to HUB) misses connections.

In this case, it appears that the decision was made to dispatch the HUB-BBB flight, which was apparently already late, so as to get it back to the HUB with the least delay and hopefully have all those passengers make connections. Unfortunately, 12 HUB-BBB passengers were inconvenienced - but delaying it further may have inconvenienced many more.

Jim