I always thought that Lady Sybil was a reference to the way that (some) noblewomen these days have forsaken the old ways - i.e. sneering at "commoners" - and actually doing something to help them. Witness, for instance, the Lady Sybil Free Hospital.
Sybil's motivations and actions stand out sharply in contrast to someone like Lord Rust. I suspect that Rust would have agreed heartily with these words from Gilbert and Sullivan's "Iolanthe":
"Bow, bow, ye lower middle classes!
Bow, bow, ye tradesmen, bow ye masses!
Blow the trumpets; bang the brasses;
Tan-tan-tara, tzing, boom!
("Iolanthe" was always one of my favourite shows, both to watch and to perform in.
It's such fun to witness lords who freely admit, for instance - in Gilbert's words - that "I don't want to say a word against brains. I have a great respect for brains. I often wish I had some myself.")
Coming back to Lady Sybil, I feel that she is a great moderating influence on Vimes. Unsurprisingly, she features prominently in G!G! and MAA, and later in TFE. She's in the background during Jingo, FOC, NW and Thud!, but nevertheless her influence is felt in Vimes's actions throughout NW and Thud! - during NW, Vimes's greatest motivation is to get back to Sybil. In Thud!, his greatest motivation is to protect Sybil and Young Sam.
Nevertheless, Sybil is hardly a Damsel in Distress that needs rescuing. As we've seen in TFE, she's quite capable of rescuing herself. As we've seen in G!G!, she can pick up a sword and charge with the best of them. She just doesn't (often) want to.