It really depends on what you're using your primary domain for. If you want people to give you money, for example, I believe it's critical that you get the misspelled versions of the domain. If it costs you $10-15 a year for a domain and you get $10-15 per download, subscription, online listing, etc. from a customer that was redirected from one of the misspelled domains, then the domain is pays for itself and reduces the friction for future sales.

The problem is having too many domains. After a while, the price adds up and the overhead isn't worth it, either. I recently transferred a few hundred domains from GoDaddy to another registrar and it literally took half a day to do it. Lock-in sucks.