Is it necesary to finance fixed assets by long-term financial resources - and vice versa?

In their textbooks, many scholars (however not all of them) hold, that firms should finance their current assets by short-term loans and their fixed assets by long-term loans, together with equity. Banks also lend money on a principle "give us all details about the financed item", at least the Czech banks do. The underlying idea is that each item of assets has its own source of financing. In contrast, accounting systems are built on an all-liabilities&equity-against-all-assets basis. Where is the rationel? This paper tries to show that the rule of thumb mentioned in the title would eventually lead to irrational behavior of the firms.