Earlier quoted context omitted.
Being able to easily check if a letter is between 'a' and 'z' in code is an advantage. This is only possible if the codepoints are sequential.
It's not a big advantage. EBCDIC, for example, didn't do that, and programmers managed just fine without it. Also, why are you doing that check? Is it to see if something is lowercase? If so, your check will get the wrong answer for lowercase letters like å. Unicode does have a way to check if something is uppercase/lowercase, when that distinction exists. This is in UnicodeData.txt.
You might try asking an old IBM programmer just how "fine" they felt dealing with EBCDIC...