If you buy insurance from a company that uses independent agents, you are paying for the commission of that agent in your premium. The agent doesn't handle insurance functions, but can provide more personalized customer service on your behalf, and that's why you are paying a little extra. The company just earned premium because of the agent's work, and thus is willing to give the agent a slice of the pie. If you buy insurance direct from a company, you save some money, but you are dealing with whomever is on the other end of the line when you need questions answered or claims filed.
Some companies service direct customers quite well and it may not be worth the extra money you give to an agent.
Similarly, there is nothing wrong with an Introducing Broker, in concept, from what i understand. I've looked into this a bit over the last few months (only been trading a few months) because I set up an account through ForexMeta, an Introducing Broker for FXDD. To be honest, I did it without really knowing what the heck I was doing. To date, I've had no particular issues with the service of FM, but on the other hand I haven't exactly been all that demanding. They add a pip to the spread on all the quotes from FXDD.
I understand the business relationships involved, so I sent an e-mail to FXDD asking for guidance on what the obligations are on their side to FM, and how I might go about dealing with them directly. It's likely that they are not allowed to solicit FM users directly. The carefully worded e-mail eventually answered the question that there is no particular obligation, and if I wish to make that move I can contact their sales area, though the same e-mail in no way discouraged me from using FM.
For the time being, I'm sticking with FM, at least until I can get some consistent success in trading. If and when that happens, the lot sizes will increase, and the extra pip will mean more to me. So, for now, I'm paying for the convenience of simply not messing with changing my platform and all that.
Another consideration some people have mentioned is that the IB does introduce yet another party into the mix as far as potential stop-hunting, since they may be taking opposing positions. I've seen no evidence of that with FM, and have no basis to suggest it is happening.
Eventually, I'll likely go FXDD direct. I'll probably give FM a year, since I have this old-fashioned sense of obligation to not just hop around over a buck or two. That said, the pips do add up over time, so it sin't a silly thought to look for a better spread. But if you're not winning your trades anyway, the spread won't be the reason you drain your account (unless you're scalping, I suppose, where spread is pretty important).