The plus and minus signs (+ and −) are mathematical symbols used to represent the notions of positive and negative as well as the operations of addition and subtraction. Their use has been extended to many other meanings, more or less analogous. Plus and minus are Latin terms meaning "more" and "less", respectively.
There is the possibility of confusing the user if you use
-. "I don't want to add another thing here..."
In addition, you may be using the
- symbols elsewhere in your UI - as a means of adding new line items, or new users for instance - so if you start using them for opening/closing containers then you're risking having the same symbol being used for two different purposes in the application. And that's a bad thing. And even if you're not currently using them, if you take those symbols for this purpose now that restricts you being able to use them differently in future.
Although really, it's not wrong to use Plus and Minus (again as Wikipedia states they are latin terms for More and Less, which could be said to apply to a 'show more' option), it's just that there are possibly better alternatives. But providing the visual design is clear, and usability testing doesn't throw up any confusion then it's probably not worth getting too concerned with.