Why bother with inputenc and fontenc?

The standard input encoding for Western Europe (pending the arrival of Unicode) is ISO 8859–1 (commonly known by the standard’s subtitle ‘Latin-1’). Latin-1 is remarkably close, in the codepoints it covers, to the (La)TeX T1 encoding.

In this circumstance, why should one bother with inputenc and fontenc? Since they’re pretty exactly mirroring each other, one could do away with both, and use just t1enc, despite its shortcomings.

One doesn’t do this for a variety of small reasons:

You’ve been happily working in this mode, and for some reason find you’re to switch to writing in German: the effect of using “ß” is somewhat startling, since T1 and Latin-1 treat the codepoint differently.
You find yourself needing to work with a colleague in Eastern Europe: their keyboard is likely to be set to produce Latin-2, so that the simple mapping doesn’t work.
Traditional LaTeX
You lapse and write something like \’{e} rather than typing é; only fontenc has the means to convert this LaTeX sequence into the T1 character, so an \accent primitive slips through into the output, and hyphenation is in danger.
The inputencfontenc combination seems slow and cumbersome, but it’s safe.