I absolutely agree with that statement. Penny's character serves as a non-nerd "everyman" that counters and thus accentuates the quirky personalities of everyone else. The character could have been male, but for the sake of the mandatory love interest (as you have noted) she was made female.
She introduces plots points or character struggles that wouldn't exist otherwise, and through these we can see the other characters' traits more clearly: Raj's inability to talk to women without being drunk; Sheldon's attitude towards relationships and "lowly" non-academics in general; Leonard's need for love and and his vulnerability when it comes to women; Howard's unrelenting desperation towards the female sex. Without Penny these idiosyncrasies wouldn't be revealed so plainly and the dynamic of the group would remain at stasis, but with her the balance is upset, and hilarity ensues. I really don't think you would have the same TBBT without Penny, quite frankly.
In that vein, TBBT's IMDB.com trivia section notes that the original working title of the show was "Lenny, Penny, and Kenny."
Ultimately from this source: Variety's Bill Prady interview.
I do think Penny also adds some variety to the dialogue, breaking up the guys' talk with her own cadence and setting up one-liners for Sheldon or saying them herself.
Gah. Well of course I disagree with this completely since I am female and find the show hilarious, plus appreciate that the science noted is real too. I can see why one would assume "women do not get the geek jokes" if your particular field is devoid of them, but the fact is that the female demographic exists and is growing in the science industries, where these types of personalities abound. That said, I don't think you don't necessarily need to be a geek or a science-y type to get the jokes. It's a well written show.
So for the record: Penny's a great and lovable everyman...but I tune in for Sheldon, like everyone else. ;-)