Have you ever tried using a blender instead of a food processor? As far as I know there are many similarities between these two appliances, but I'm not sure if they can be used for the same purposes. I own a Ninja blender, which has a high-speed motor. It does a great job at blending, grinding and even crushing. In the beggining I didn't know How to clean the blender, but this job is very easy if you follow all the instructions. Anyway, is it worth buying a food processor if I've got this modern blender?
There is a lot of overlap but also some differences between the two.
A high speed, multiblade blenders like Vitamix and their consumer adaptations like the Ninja are far superior for blending smooth and consistent liquids over food processors. I have a Ninja, which my wife uses far more than me, and it makes soups and purees smoother than my food processor. No matter how long I run my food processor I can't get the same consistency.
My food processor is better with dry ingredients and pastes than the Ninja. It creates a more uniform consistency with dry/semi-dry ingredients and ingredients which are oily (e.g. nuts, tahini). Many also include attachments that can slice, grate, etc. A food processor is more of a food prep tool than a tool to blend consistent liquids. It's more like a pair of hands with knives chopping at an extremely fast speed than a tool to puree.
Unless you desire the attachments of a food processor I don't think one is absolutely necessary if you have a multi-blade blender but it depends on what you cook and how exacting you are about prep. I have a Ninja, food processor and immersion blender which all have overlapping purposes but get used for completely different purposes.