In the old days during the dawn of television, everything on the boob tube was live. A lot of those live dramas and comedies were never recorded for posterity (or if they were, the recordings were eventually taped over). Like theater, those moments are lost to memory -- fleeting performances that happened once, never to be experienced again.
But television evolved into something more than just theater broadcast into people's living rooms. Just like film developed its own language, so too did television. So when those recorded series try to do a live episode, it usually falls flat, failing to be anything more than a novelty. Such live episodes are usually unwatchable for repeat viewing.
TV shows have experimented with live broadcasts -- Gimme a Break did it in 1985 and ER did it in 1997. Other series that have offered live episodes include The Drew Carey Show, Will & Grace, and The West Wing. In 2000, George Clooney starred in a live televised remake of Fail Safe. To be honest, none of those live productions is very memorable, because being live did not add anything worthwhile to the experience, but instead had the effect of making the broadcast look amateurish.
For some things, being live adds an element of excitement and immediacy, like award shows, ballgames, and news -- but even those often have slight tape delay and pre-recorded content. With Digital Video Recorders, people are taping even live shows and watching them later. So is there a purpose to live television anymore?
We watch live events hoping for the unexpected, that "did you see what just happened" moment that might never be seen again -- but it is not live stage theater. We will see the replays, we might record it ourselves and rewatch it. "Live television" is almost an oxymoron. Experiments like 30 Rock have not proven that it is anything more than a weak gimmick.