It just makes financial sense. I watch the holiday movies on the Hallmark Channel and they use the same houses in almost all of their movies!

and I think it's great that they did it that way! They probably saved a lot of money. If they're working with a tight budget, then this is the way to go. At least, it WAS the way to go back when people couldn't record the episodes in order to double check these things.
No big deal. Since there are houses which are similar to each other in reality (especially in certain neighborhoods), the idea of those characters living in houses which look alike isn't so out of the ordinary.

Jim Hutton (1934-79) and Ellery Queen =

They did it on Gilligan's Island, too Gilligan goes over the side of a cliff and dangles from a branch growing out of the rock (wearing his white sailor hat) a few episodes later, he is wearing the Skipper's hat, going over the cliff, but because they used the same shot, he is magically wearing his white hat a few seconds later when you see him dangling.
They did it a few times on The Ghost and Mrs. Muir (series) too. They had one establishing shot showing the house from a distance, which in itself, was a re-touch the real house, located in California, not Maine, had no ship's wheel, no widow's walk and no stone lions guarding the house. but DID have twelve stone steps leading up to the front door. The items were were airbrushed in but the GAMM set had only two stairs leading up to the front door.
Same shot was used all the way through the show.
I've seen the EQ eps countless times never noticed it was the same mansion.
How sad, that you were not born in my time, nor I, in yours.

They could get away with that sort of thing back then because there was no way for people to record the episodes and double check the scenery later.

Jim Hutton (1934-79) and Ellery Queen =

Indeedthere's a shot of three extras in Star Trek that is used in two different episodes.

I'm sure that this sort of thing has been done other times, especially when the studios are really trying to save money.

Jim Hutton (1934-79) and Ellery Queen =