Which of course sparks an entire debate. Creators of shows that start with UST seem to want to avoid resolving the UST for as long as possible - sometimes for the entire run of the show. They point out numerous examples of male and female leads that start off as "partners" only for work and when the show gets them together romantically - poof! The show "jumps the shark".
For me, the most vivid examples are The Xfiles, Moonlighting, and Remington Steele. Those shows are always thrown out as examples of keeping them not a couple, cause when they did get together - "it ruined everything" they say.
Then the shows, I'm thinking more "police drama" here. McMillan and Wife - already together. Hart to Hart - already together. It seems like when you do a police drama, they try to keep the leads - "free" romantically - not tied down to anyone. I remember back on the Rookies - one of the guys was married (Kate Jackson). Since she was a nurse, they incorporated her a lot - but then again - she was victimized a fair amount, too. And, only one of the three "rookies" was married.
Anyway - great topic that I could go on and on about all day and I'm always interested in hearing more views and examples on this.