The problem with "I'm just being honest" is you can be honestly wrong.  "Being honest" doesn't inherently make you right and therefore it's more like you are just putting a large value on your opinion instead of helping people out.  Obviously this isn't always the case but it's why you should be cautious when "just being honest".

Oh....

And before you reply I just want to say I'm "just being honest here".   (See how that works?)  Nobody is special because "they were just being honest".  It's a phrase anyone can use without discretion for truth.