The Sox this year have attempted 104 stolen bases and been successful 87 times. That's an 84% success rate. If the break even point is around 70% statistically the Sox should be running MORE. And they're not the only ones:
LAA - 83%
Brewers - 80%
Cleveland - 80%
Baltimore - 78%
Washington - 77%
CWS - 75%
Cincinnati - 74%
Houston - 74%
NYY- 74%
Arizona - 73%
Atlanta - 73%
Minn - 71%
KC - 71%
Texas - 71%
Colorado - 70%
Assuming that the 70% success rate is correct then 16 out of the 30 MLB teams - more than half - are at least breaking even on their steals.
The problem with statistics in general is that they assume that what happens once will happen again. In the case of stolen bases it's probably safe to assume that if these teams attempted more stolen bases their percentage would go down because they'd be stealing in situations with a higher opportunity to be caught.
Here's why you can't say that attempting to steal a base is either good or bad: Like in life, if you're good at something it's a good idea to do it, and if you're not good at it it's a bad idea. However, attempting to steal a base per se is not a bad idea.