Yes it is a training aid, and that being said, largely depends on who it is that is using it. But the problem I find is that the people who are more inclined to use them, are the ones who really shouldn't be.
I personally dislike using any sort of training aid except as a last resort. I will use side reins while lunging every once in a while to help build topline, and, coming originally from a hunter background, I used to think I should be strapping standing martingales on any horse. 5 or 6 years ago I came to the realization that if my riding was correct I shouldn't need one, and stopped riding in them all together in order to really improve my riding.
There is, however, one horse who I ride over the summer that almost everyone uses a standing martingale with. For the longest time I wouldn't use one with him, and on the flat there was (is) no difference in him while I'm riding with or without martingale.
When jumping however, he has the tendency to get excited and throw his head up attempting to avoid aids. Now for the longest time I didn't put much thought to it. He was perfectly controllable, just made things a little more difficult then otherwise. I wasn't too concerned however, because we are very solid on the flat and he listens very well to leg and seat aids.
Well, one day, I hopped on him to run a course after one of my friends had been jumping him with a running martingale. I was giving him back to her (I just wanted to try out the course cause it looked fun xD), so I wasn't going to bother taking the martingale off or anything. But when I jumped this course, there was a very noticible difference in the way he went. Because he couldn't throw his head in the air every time he saw a jump, I had a much easier time with him and getting him to relax (a little) and concentrate instead of simply getting all excited. After this, I started to use a martingale with him every time we were doing work 3' or up.
My problem with them, is that I find people grabbing them and putting them on any horse just "for safety", or because they can't control the horse otherwise. My opinion is that if you can't ride the horse without the training aids, you should not be riding them at all. Even worse, I see people jumping in standing martingales. I know this is common practice, I used to do it myself, but I now understand how dangerous it can be, and it really bothers me.
Sorry for the novel, but the question is, what do you guys think about running martingales. Do you think it's right for me to use one just to make things a little bit easier? The end result is always the same (almost always a very good one) whether I'm using the martingale or not, the martingale just makes it easier to get there.