If you have no access via (s)FTP, or at least some hosted file manager, how did you get the site uploaded in the first place? And how are you going to upload backup files if things go bad?
If you have no access via (s)FTP, or at least some hosted file manager, get a new host.
If you break things with the built in editor, and can't get into the site by some other mechanism, you can't fix it. That is the problem. That is why it is dangerous.
No hostility here. I'm not trying to pick a fight but it is possible-- in fact it is very, very easy-- to break a site badly enough that you can't get back into the site to the built in editor to fix it, even if you could remember what you did to break it. Try leaving out a quote mark, just one, in your wp-config.php file, or in your functions.php, or mispell a function name somewhere...
Plus, the editor makes it very easy to tweak files that haven't been backed up, despite the warnings.
You do what you want. It is your site. It is your choice, but it will bite you or it will bite someone else who doesn't know any better, who doesn't realize that you can take down the whole site with forgotten punctuation or bad spelling. This is for those people. Be careful with this thing. At least know how to connect over (s)FTP before you mess with things.