Answer:
Depending upon the application or OS in use, this will result in lack of functionality, or extremely poor performance. Just having the minimum memory required for Windows, for instance, will lead to excessive paging to the swap file (where data in memory is dumped to the hard drive to make space in RAM, then swapped back as required), which greatly reduces performance as a direct result of the lower access speeds of permanent storage compared to RAM. Having less than the required memory for a given application may result in the application refusing to run, or running with greatly reduced performance - it may even cause lock-ups and unresponsiveness.