Running one (or even more) defensemen short is a viable strategy that is used by some managers out there. Technically, it’s not against the rules unless your particular league has a specific rule that stipulates you must fill up your starting slots. Otherwise, in general leagues, like Yahoo! public leagues and whatnot, you can have as many or as few players at each position as you like. There really doesn’t need to be a specific rule against this, because it makes it inherently more difficult to maintain a team when you’re not using up all the default spots; basically, it doesn’t automatically help you. I can see how some people would be put off by something like this but I personally don’t see a problem with it because of the cost/benefit nature that comes into play. If you know what you’re doing and you think you can make it work then it’s a definite option out there. Also, this is NOT like streaming.
Obviously, if you’re in a league with max games then this strategy will do much more harm than good. It is, however, doable in a head to head league.

Example Team: Defense Group
Now, in the team above, you can see that the defense core here isn’t exactly top notch. They’re not all terrible, though, either. Depending on who’s out there in free agency, it’s feasible to say that one or two of these D can be dropped in favor of a couple forwards. In the long run, it’s highly likely that the forwards would accumulate more fantasy worthy production than the two weakest D on the roster. Also, because there is a util spot available, that should alleviate the need for benching.
The premise is based on the fact that, on average, forwards score more than defensemen do. So, depending on your league categories, it can be beneficial to run one d-man short in favor of an extra forward. Over the course of the season, if done properly, the lost production from that missing starting blue line position should be offset by the higher average production from forwards.
One of the most obvious problems that can arise from this is having to bench players on busy nights because you would have that extra forward. This won’t always happen but it can and will be an issue on the busier schedule nights in the NHL. If your league uses util slots then you’re going to have less of a problem because you’ll have the luxury of being able to start that extra forward in that util slot when available.
In the long run, if you’ve chosen good a replacement forward, you should be able to squeeze more production out of that extra forward (even if you have to bench them on occasion) than you would out of a weaker blue liner. Of course, if you had strong defensemen to begin with, then you wouldn’t be thinking about doing something like this, anyway. This would come in handy most when you’re perhaps in a little bit deeper league and you know that your depth defensemen simply won’t be that great. Unless you specifically tried to draft or acquire very strong D, it’s likely you’ll have mediocre or downright poor production from the bottom couple D in your group.
Personally, I like to have a balanced team (with strong D) so I don’t really utilize this running D short type strategy myself, but I have seen it done before and many people do use it if it suits them. It can be effective if done under the right circumstances. Again, every league is different so tweak whatever general strats are out their for your particular league settings and team make-up.