The double edged sword of modern television. Because you don't know if you are getting another season, writers have to have an incentive for those "shippers" to keep watching.

So which shows did rush the main characters together and was it for the better or the worse?

Moonlighting. After they got together, it went downhill fast.

Rhoda and Joe on Rhoda.

I hate it so much how the Odd Couple (70's version) ended.

Not sure if these qualify, but I think most people feel that the final season of "Three's Company" really rushed the romance of Jack and Vicki, to facilitate the new "Three's A Crowd" series.

Also, the Chakotay/Seven of Nine romance in the final few episodes of "Star Trek: Voyager" was completely out of nowhere and terribly rushed (as well as flawed).