我是不是只见树木不见森林？
Delphi 在inifile.ReadString()上返回''，而不是提供的默认参数。根据帮助，当节、键或值丢失时，它应该返回默认值。
我很肯定我以前用过很多次，没有任何问题。

unit Unit1;

interface

uses
  Classes, System.SysUtils, Inifiles, Dialogs;

implementation

const
  DRIVE_SECTION = 'USB_Drives';
  DRIVE_LETTER  = 'DriveLetter';
  INI_NAME      = 'c:\temp\test.ini';

//--------------------------------------------------------
procedure CreateDefaultInifile(const IniName: TFileName);
var
  slist: TStringList;
begin
  slist := TStringList.Create;
  try
    slist.Add('[' + DRIVE_SECTION + ']');
    slist.Add(DRIVE_LETTER + ' = ');
    //
    slist.SaveToFile(IniName);
  finally
    slist.Free;
  end;
end;

begin
  if not FileExists(INI_NAME) then
    CreateDefaultInifile(INI_NAME);
  // now we have an inifile with a key that has no value
  var ini := TIniFile.Create(INI_NAME);
  Showmessage(ini.ReadString(DRIVE_SECTION, DRIVE_LETTER, 'Missing_Value'));
  // according to Delphi's help, the default ('Missing_Value' in this case) should be returned if no value is assigned!
  ini.Free;
end.

end.