I think it really depends on where you are. For us, a small hostel in rural Nova Scotia, it creates an income for one of us. The other still needs a fulltime job. Since we live in our hostel though it gets interesting: home based business tax deductions..... (In other words: by the time I'm done with all our deductions our official income on the hostel is 0, but it does pay for our mortgage, property taxes, utilities, and then some). For us not worth it if you wouldn't live in or if you'd try to make the hostel the sole income.
Before buying antyhing it would be a good idea to contact other hostels that might already be there in the city you're looking at: ask them their occupancy rate and if they think there is space for another hostel. Most hostel owners would be honest about it - if they're sending people away certain times of year or if they would rather send their overflow to a nicer hostel then what is there now, they'll tell you. And even if they end up not wanting to help you're no worse of then before!