Frankly, there is no way I'd change a end boom sheet to a mid boom. Same reason - more control and less force - not to mention it makes single handing that much more difficult.
Some boats are designed for a mid-boom traveler, others for an end boom traveler/sheet, and most somewhere in between. The forces on the deck or cabin top transmitted from the mainsheet are tremendous, and must be located at a heavily re-inforced location. To put a traveler (or worse yet, just a mainsheet block system) on top of a cabin that was not designed for these forces is asking for trouble. At best, the cabin top will flex and crack, as worst, you will suddenly have a "convertible" sailboat when you are trying to go to weather on a windy day. If you are making that conversion and installing a traveler, at least you will be distributing the force over several feet and multiple points. Don't try to do it without a traveler, and even then only if your current boom and cabin roof/cockpit/seat can handle the stresses. Sailboats, even large ones, have a tendency to "bend" in a bad way if subject to forces not intended for their structure.
The other issue as addressed in several other posts, is that the traveler is a critical sail handling tool. It is what allows us to maintain optimal mainsail shape for the prevailing conditions. Most of us that race or try to optimize sail shape use the mainsheet to set the sail shape, and the traveler to adjust the main to the appropriate position, including reducing weather helm, depowering, or shifting gears going to weather. It has much less function downwind, but still can be useful. While I'm not sure I value it next to a "good, powerful vang," I can't imagine trying to sail a larger boat well without it.