Trouble arises when the system in question does not have enough performance overhead 'get around' the issue. Textures may fail to load or 'pop in' after the rest of the area has loaded, the game may hitch or stutter before recovering, or, in worst-case scenarios, the game may unceremoniously crash to the Desktop.
On powerful PCs, this may not be an issue, as the resources available my be able to 'get around' the limit by simply processing calls 'later'. As players install mods, it pushes this limit closer and closer to the point that it may run out of RAM. This is not governed by the amount of RAM on your system, but by the amount of RAM that the engine itself is coded to use. Most notably, there is a finite amount of memory (RAM) and other system resources that any engine is capable of utilizing. This is not an official response, but based on the growth of modding overall (courtesy of games like Bethesda titles, Gary's Mod, Minecraft, etc.), mod limits are imposed for a number of reasons.