If you develop WordPress websites for money, there should be no problem with using a premium themes. I have fixed many sites that had problems with free themes after the theme author abandoned the theme. At least with premium themes the authors have a reason to continue development and support. I avoid free themes for client sites. I’m not saying that all premium themes are trouble free, just that these themes have fewer problems since there’s a financial incentive to keep them working well.

That being said, premium themes being offered as “free” themes should be clearly identified as “limited” versions. I do like being able to evaluate these limited versions to determine if I even want to purchase the premium version.