@blackvax7:
If you saw above she said she wants to leave. She hasn't said why yet(probably find out later)
Yes, but that's her saying she wants to leave because she feels guilty for helping Doflamingo, and thinks the people won't forgive her. She isn't going to be forced away from the family she recently reunited with.
and Navigation and lookout are 2 different things. She has a limit of how far away she can see. (and Dressrosa is relatively small.)
Not in this case it isn't. You're talking as if Viola's sight radius is small…. when in reality it's 4000km (2485 miles), which is not "small" by any means or that much of a limit at all.
If you look at it the right way she could possibly help the adventure aspect. She'll see something in the distance that luffy could be interested in prompting them to go on an adventure.
Again, it really wouldn't help. The main appeal to the Straw Hat's journey is that they really don't know what to expect next in terms of weather, places, and things. Viola's ability would be able to see all of those things and ruin the surprise for the characters, which in turn ruins the experience of the reader.
There are other reasons I could list other reason for her joining besides these if you'd like.
You can write down all you like, but the main deterrent from Viola joining the crew is her non unique personality and design, and overall lack of screen time in her introduction arc.