I've been working with vbulletin a lot recently, and you're able to change style templates from the ACP because that's how it's set up. This would be great for phpbb.
Yes, ftp is there and available and easily accessible. But sometimes, you need to make a quick fix or change, or you're only working on one page of a template for a little while, and for these situations, at least for me, native template editing support would help. I'm sure there are other scenarios.
Editing templates for extensions would be even better but I'm trying not to get ahead of myself here.