Operating cooling systems at pressure is done primarily for two reasons. Firstly, it raises the boiling point of the coolant. This reduces formation of bubbles which would otherwise reduce conduction of heat into the coolant. Secondly, it helps reduce cavitation at the coolant pump. Cavitation reduces the pump's efficiency.

Remember too, that these cooling systems were designed to operate at pressure with the thermostat regulating temperature to relatively high values, close to and even above the the boiling point of water without any additives. So long as your are aware of these issues while driving with the pressure cap released, and drive the car accordingly you should be OK. I would suggest avoiding high ambient temperatures, high engine loads, high engine speeds and use of the air-conditioner. Use of the heater can also help reduce the coolant temperature.

I remember many years ago, "nursing" my 350SLC back home over several hundred kilometres with a badly leaking coolant pump. I too, removed the pressure cap to reduce the coolant loss. The problem was that on any uphill climb or with any attempt to cruise at speed on the freeway, the coolant wanted to boil, all aggravated by summer temperatures at the time.

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107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.