My 25 foot line is sometimes about 5 feet shorter than I would like, but you certainly don't need 100 feet.
I have found that the length of the line needs to be adjusted as the speed of the mother ship varies, to keep the dinghy sliding down the wave--speed of the boat affects the following wave. As well, I always pull the dinghy closer when approaching the dock or navigating in a narrow channel. So it would be helpful to have some way of adjusting your line while underway.
I have found it works well on my E30+ to tie the line to a stern cleat where I can easily reach it from the helm to make adjustments (just adding a few wraps around the cleat will bring the dinghy closer, or in calm conditions, I can tie it off to the stern pulpit/pushpit as I approach the dock. I tow an 8' Walker Bay fibreglass dinghy, weighing 70 lbs, which exerts minimal drag on the mother ship. I know the preferred method is use of a bridal, and if I were going offshore, rather than coastal cruising, I would probably use one, but for me the stern cleat works well with no apparent downside.
Frank