Is it a problem? As it is, characters gain heavy armor proficiency and a feat. Any character who does this is giving up something, whether it's a level of spellcasting, a sneak attack or special rogue ability, or a stronger animal companion. While it may often be advantageous, keep in mind the Fighter has only one good save and a fairly limited skill selection. Taking one level of fighter says, "I woud like this character to take the brunt of combat from time to time," and to do that, you need a good AC.
The biggest issue I see is for rogues, who go from light armor to heavy armor, or in this case from slow progression to fast, while retaining advantages of their rogue abilities that don't normally mix with armor and getting a Fortitude bump and better weapons. On the other hand, they take the biggest hit in the skills department and that puts Improved Evasion or Crippling Strike one level behind.
