I don't expect realism from Spielberg. I expect a heartwarming story, and focusing on kids and animals (or cute aliens). That's pretty much been his MO his whole film career.
I wouldn't spend money to see War Horse at the movies, but I do have it on my NetFlix queue when it's released on DVD. I expect to like it, even while cringing at the unrealistic aspects.
As far as Seabiscuit or Secretariat being a 'rip off' on Phar Lap, I don't know how anyone could make that even into a tenuous connection. All 3 were real horses, and their stories weren't similar except that they were all TBs, and all race horses.