It is a good solution economically for people who have a 970 already. You can accept the increased heat/noise problem in return for good FPS on many games.
However there are titles where they don't scale well enough or very well at all. Here it becomes a problem. This is happening less and less, however it is still a regular occurance outside the top AAA titles.
there are many benefits and less hassle and running costs with single GPU solutions.
What I would not recommend is buying two cards, only adding another if you already have one instead.