Should every TEXT field of schema RichText enabled? We use title, headline etc. as text field, can we have these as richtext ? Is there any issue having all the text fields as rich text enabled field?
I would say that you should use Rich Text fields sparingly, because it gives authors control over formatting/layout/presentation whereas you want to keep as much as possible of that in your templates/views.
But that's the "philosophical" perspective. Technically, there is nothing stopping you from making all text fields Rich Text fields and you can also restrict the markup that can be used within a Rich Text Field in the Schema (ideally, you restrict the markup to the more semantic HTML elements).
Besides everything that Rick explained in his answer, which I completely agree with, I would like to add that using RTF fields in system can be different then using "normal" single line text field.
For example, RTF field class in TOM.NET API is inherited from
TextField class, so how you handle them in code can vary and can have certain disadvantages and unwanted repercussions.
So yes, please use them least possible, in controlled scenarios where you can ensure that it won't break layout of your template.