NOTE: All manufacturers designs are different, furthermore each model is different. So one boat the middle seat might be holding the thing together, where another brand or even model in the same brand wmight be totally different. It is a real sticky situation to just start taking seats and floatation foam out of a boat, even when it doesn't seem to make a structural impact. Most places I have read (including manufacturers) say do NOT take them out. Part of me thinks it is probably just to avoid lawsuits, but looking at boat designs, almost every one that has no middle seat has some sort of full gunwale structure on each side to make the boat rigid. So another part of me, particularly the part that thinks about the times I take my wife and son on the boat, says I can work around them.
I had the same desire, and in some ways still do to remove the middle bench. But for me the risks far outweighed the desire to have no middle seat bench. If you do take it out be sure to test the structural integrity before loading it up and heading out. The real problem with these kinds of situations is you normally don't see the situation where too much stress is there until it is too late and the gunwale splits wide open or something freaky like that. Different situations particularly when dealing with a structure that is not on a foundation i.e. floating in water make the structural needs almost changing with every wake we hit, or current rip we enter. It is your boat, so ultimately weigh your options and do what you want. Everyone here will offer our opinions, but in the end it is your boat that you will be in time and time again on the water.