How can one say they're roleplaying if they've never roleplayed a personality other than their own?
You are making a lot of unwarranted assumptions here. For one thing, there's roleplaying oneself in a different situation ("What if I was a musketeer?"), roleplaying a different character with one's own basic personality ("What if I was a lean, mean fighting machine?"), and roleplaying a different character with one's own personality in a different setting ("What if I was a musketeer who knew how to fence?"). How can anyone say that that's not roleplaying? For another, who says preferring a rules set that provides tools to design different characters to play means that they can't make do with a rules set that doesn't provide such tools? For a third, how can anyone say that one is more likely to play a Traveller character with a different personality than a GURPS character? The Traveller character comes with a complete blank for a personality, which allows one to provide it with any personality one desires, but which personality is the easiest one to come up with? That's right, one's own.
How can they say they didn't already know that physical attributes, personal traits, and personality quirks were always part of a player-character? Why did it take a rule book to open their eyes? Why should it have taken a rule book?
Who says it did? And who says everybody played their Traveller characters with personal traits and quirks? Maybe you gave your very first character a limp that made him unable to run fast, a squint that gave him a minus to his markmanship, and a craving for marshmallows the referee could use to get him into trouble, but I very much doubt that you were in the majority. No way to find out for sure, of course, but I certainly didn't. Yet I remember a lot of good roleplaying; just not right out of the box.
My first ever RPG character was a low IQ fighter in three book D&D. I didn't need rules to tell me that playing him as a helpful, eager, and sadly dimwitted trouble magnet would make for great sessions. Rather than waiting for some rules to tell me what to do, I used my imagination instead.
My three first ever RPG characters (D&D, natch) started out with the personalities "Lawful Good", "Neutral", and "Chaotic Neutral" (putting them one up on my first Traveller character, which didn't come with any personality at all
). The first adventures were pure dungeon-crawling. Much more a game of maximizing the effectiveness of our characters' combat abilities than real roleplaying. Gradually, they developed individual personalities. The LG fighter started to resemble Blake from Blake's 7, the others developed character traits of their own. I didn't have GURPS at the time, nor did I need it to role-play. But a point buy system would almost certainly have gotten us off to a much better start.
The point buy system I eventually developed for myself has one purpose and one purpose only: To make my players think about their characters in advance. Once the character is created, I throw away the point sheet -- character development is handled in an altogether different fashion.
GURPS didn't introduce advantages, disadvantages, and quirk to let gamers roleplay better, those things were introduced to allow gamers to use the points associated with them and build more munchkin-like characters.
That's quite an accusation to make. Steve Jackson and his evil cohorts actually sat down and figured out a system for the sole purpose of getting players to behave like munchkins?
As one poster in this thread already pointed out, GMs rarely hold players to full effect of the disadvantages they choose.
What a devious guy, that Steve Jackson. He deliberately designed his system to subtly influence GMs to ignore the rules.
As for quirks, there are regular thread at SJGames discussing whether proposed quirks are "quirky" enough or whether they're just another way to pad a character's point total.
Effectively quirks just mean that almost all so-called 100 point characters start with 105 points. Big deal.
Hans