note: here I use the convention:
short term: < 1 year
short long term: 1-10 years
long long term: 10 years +
Even if you are positive in the "Long Long" term it may not work out that way in the short long term (yes, yes I know) and so you hedge against short term losses due to volatility etc. with short positions (usually options). Short term losses include crashes and corrections and so in the short long run (a crash is long run for traders, short run for investors) you can profit from both being long and short as you profit from the fall in the short long run (the crash and following depression/recession) and from the long position in the long long run when prices recover. Long long term short positions are there to cover the possibility that the company makes poor decisions or goes bankrupt (or a few other things) and the stock falls to a sustained low.
The exact proportion of long positions to cover with short positions is a difficult calculation and there are many long books devoted to "portfolio theory" covering the intricate maths behind it. In short the short positions of a portfolio should be enough to cover any short term volatility in the stock price (measured by historic volatility plus a little bit of statistical projection). There is not and cannot be any general good rule for this; a very volatile stock will need a much larger hedge than a less volatile stock. Long long term hedging is related to the probability of default of the company and results in small short hedges.