Oh boy, it's my time to shine. This is going to have a pretty long answer...
When you're talking about the rulebooks, you need to remember that they were never made for any kind of tournament play, and so points aren't really a consideration. The 'draw' result wasn't clearly defined by the rulebook for some time due to this, and although more recent rulebooks clarify it a little, most face-to-face tournament directors ignore the scoring section completely and instead use scoring systems developed by other tournament directors. You'll also see (if you read all this) that no online site follows any rulebook correctly either - we'll come to that later.
The 1959 Rulebook (the very first edition) specified that 'If no player gets a majority [of the pieces on the board] during the time set aside for play, all players who still have pieces on the board draw.' It's definitely considered better than a loss, because it's specified separately that eliminated players are considered to have lost in the event of a draw, and it's also clear that it isn't as good a result as a win, as the win condition is also stated separately - but that's as specific as it gets.
It should be noted that at this point, the win condition for the game was having the majority of the pieces on the board - not 18 supply centers. You could have 18 and not win if you hadn't built enough units, and likewise you could win with fewer than 18 if other people didn't have enough units on the board. This was the victory condition until the 1971 Rulebook changed it to 18 SCs.
However, that initial reasoning that pieces are what matters, not Supply Centers, is still followed in the draw conditions of the more recent rulebooks. But I'm jumping ahead of myself here - let's move on to the next edition, in 1961.
The 1961 Rulebook removed the concept of a draw altogether, probably because of the ambiguity and the fact that Calhamer never liked the 'Carebear' approach that including the draw result caused. It stated simply that either the game would be played until a player won in the normal way (having the majority of pieces on the board) or a variation called the 'Short Game' could be played, in which case after an agreed number of years, the player with the most pieces on the board would be declared the winner. This was the official ruleset for a decade, until the 1971 edition.
The 1971 rulebook reintroduced the concept of a draw, and was the first rulebook to be more specific on what that meant - by specifying that players with pieces on the board share *equally* in a draw. This is where what we call Draw-Size Scoring originated, although webDiplomacy adjudicates it slightly differently, using supply centers rather than pieces.
However, this rulebook also keeps in the idea of the 'Short Game' variation as a separate option, with exactly the same rules as 1961. To see why that is, you need to look at the ways the game was being played at the time - Calhamer had originally designed it as a Play By Mail game (as made clear in his RealPolitik rules from 1958) and although the rulebook had been changed before its release in '59 to make clear it was a face-to-face board game, presumably to aid its sales, it was seeing a huge amount of PBM play. As such, the rulebook needed to account for both PBM and FtF play - PBM would almost always be played to a stalemate or victory, and FtF would be played to a time limit. Draw-Sized Scoring made little sense for a Face to Face game, because under a time limit, players would have to decide whether to play for a better draw (by focusing on eliminations) or for their own victory (by focusing on picking up supply centers) - so DSS was put into the rulebook for PBM games, and the 'Short Game' rules were kept for Face to Face.
The 1971 rules for draws/victories were kept the same for 3 more versions - 1976, 1982, and 1992 - before finally being changed in the most recent edition, 2000, to remove the 'Short Game' variation. As such, DSS (with pieces considered instead of Supply Centers) is the only 'official' drawing system at the moment.
Technically, nothing after the 1959 edition specifically mentions a draw being separate to a loss, but they do state that every player with pieces on the board shares in the draw - which implies that the draw is more valuable than the result attained by the eliminated players.