Well, when you think about it, time travelling back to the 20th century makes more sense than just coincidentally discovering an M-class planet that is an exact duplicate of earth.
In any case, time travel is a really powerful concept; but it is only powerful if the viewer is made to actually realize how huge it is. One of Trek's weaknesses is trying to tell very complex stories with very little time; so in the end, the viewer gets the feeling that they zapped back (or forward) in time, and then they cruised on back to wherever they started and - voila - nothing super interesting about that.
All in all, time travel is great, and I like all the time travel episodes. As with many episodes, however, there is always the problem of "saving the universe and all past and future time" coming across as way too much like "another day at the office." The element of absolute terror and human wierdness that would ensue if real time travelling were taking place is just simply left out.
Two simple examples; if a real life man relived his entire life as Picard did in "The Inner Light", he would be seriously messed up. Also, if normal folks travelled back in time 300+ years as the Enterprise Crew did in The Voyage Home, they would be too confused and shaken up to just climb out onto the grass and wander around San Francisco.
I have been watching the TV show "Lost" which, over the course of several seasons, tells the story of an electromagnetic anomaly and other mysterious events on an island. The story is fascinating, but as I watch I can't help thinking about how the entire TV show's plot - if it were Star Trek - would be condensed into one 45 minute show as a sub-plot, detected, analyzed, and taken care of by scanners and other equipment on the Enterprise, while Data and Geordi romped through a mal-functioning holodeck.