It's definately possible to have some success playing whole matches without defenders. As I'm waiting for the patch before starting a proper career game, I've tested some ultra attacking tactics with Arsenal and convincingly beat a Bulgarian opponent in the UCL qualifications, and in the Premier league played a few even matches. I didn't win any of them in the end, but I did always score the first goal(s). With some tweaking (or simply changing your tactics to more defensive ones, once you're in the lead) I'm sure you could turn those matches into wins.
The point is that, although Arsenal have brilliant players, I don't think any professional team should have any problems beating a tactic with no defenders, and easily. The opposition forwards really aren't able to use the space they get and tend to move into offside positions (over the halfway line) or dally on the ball when there's no need for that as the closest defensive players are more than a dozen yards away from them.
It really makes you wonder about the realism of the match engine even when using normal tactics. As I indicated, this obviously isn't a super tactic, but the results should be a lot worse. Here are the pkm and tactic files from a 4-4 thriller against West Ham in case anyone's interested:
PKM Tactics
BTW, I would have continued testing these weird tactics, but my computer crashed during the game (non-FM related crash) and I can't be bothered starting all over. Maybe it's better that way as it can ruin the enjoyment seeing unrealistic tactics work (to some extent).
0-5-5 
Linear Mode
