You're right: there's no technical reason why you couldn't do what you're asking. When users start to add content for a site, they're now content contributors. And it can certainly make sense—depending on the site. When I think of "content sharing site", Craft could be used like a Wiki, for example.
The short answer? If you find yourself reinventing what Craft can already do, just give someone access to the CP.
A longer answer and reasons not to do it? Control. User experience. And trust/security all come to mind.
I just created a site for a client where individual teachers are able to post their class schedules and such and people are able to buy those classes (via Stripe). The client loved the CP. All the teachers were impressed about how easy it was to add their schedules. The CP was definitely a plus in this area.
However, the client properly scrutinizes everyone who wants to use the site (and their credentials) and there's a training period where we show people how to use the site, where to login, etc. Your use case might be a little more streamlined if you have computer savvy users, etc.
While Craft is straightforward, if not downright intuitive, the CP might be overkill for someone just casually posting content once in awhile. There still is a lot going on. And it really depends on what content you're managing.
From a traditional web context, the CP is a still little "disconnected" from the actual website. You still have to find the page you want to edit, or create a new one, preview it, and publish. (That's why, IMO, things like "in-place" editors on are still popular with other CMS.... though I hate those things!)
Having the CP be disconnected is partially due to security. If you trust and properly vett your authors, or there's not much on the line, I see no reason why you couldn't open up the backend.
As always, take good backups in case you have to roll back to something. That's true of any server, though. No software is perfect; if there's a vulnerability found for, say privilege escalation where a user could somehow become an administrator, the more users that have access to the CP, the more chances someone has to breach the site. Craft is relatively niche so you won't have the brute-force drive-bys that something like WordPress has. But as a site administrator, you can never be too vigilant.