ok, yea your hoses arnt going to explode in a fantastic eruption, and your radiator wont detonate, but it will cause problems. with the exception of aftermarket radiators, Fluidyne, etc, radiators are poorly constructed.
a radiator is made up of a core, and 2 tanks. go to your car and look at your radiator. the core are those tiny ribbon looking things in the middle. coolant flows in those tiny passages to have extra heat taken away by air passing through it. those passages arnt that thick. its aluminum, they're pretty weak too. how many people have some of those ribbons slightly bent from rocks, or a dropped wrench of some sort? those bends create weak points in the material. if you cause the system to build more pressure before being released, you risk rupturing those weak points, causing coolant to leak out. yes leak, not gush out and spill onto the ground, but leak. going unnoticed, the leak allows air into the cooling system, and can damaged the engine by causing bubbles.
the 2 tanks on a radiator are of the same weak aluminum, but much thicker. to save on cost and time, manufactures attach the tanks to the core by means of sodder. yes the same stuff used in electrical wiring. its 3 times cheaper and easier than welding, and produces close to the same results. the downside to sodder you ask? its fairly weak. after a radiator is assembled in the design phase, the manufacturer no doubt tests these radiators. makes sense to right? and they test it and find that the OPTIMUM psi in which the radiator can hold is 15 psi. the people who made the radiator are telling you how what is supposed to be at for maximun efficiency without leaking. yes, there is some leeway. i bet the radiator can hold 16, 17, or even 22 psi, and will still hold, but coolant is corrosive. dont belive me? wash your car in stright engine coolant, no water and see how your paint looks. the coolant over time eats away at the sodder, and the water does its share of rusting the inside, making the radiator even weaker. and you still want to put more pressure inside of it?
oh wait, i almost forgot the heater core! made the exact same way as a radiator, the heater core lives under the dashboard typically above the glove box. now, knowing how "cheaply" constructed these things are, would you want to increase the risk of having that heater core leak, and get coolant all over that lovely "Civic Type R 100% authentic" floor mats you bought on E-Bay for 200 bucks? no.
yes spoon makes some great products, but their primary focus is on purpose built RACE CARS, whose cooling systems are DESIGNED for higher psi's, and guess what, race cars dont have heater cores, so if you buy an coolant cap with a higher than stock rating, feel free to also buy a new radiator, heater core, and various other componets, just because they will fail much sooner than normally.