If I uninstall the exchange server everything will be reverted to previous state?
No. Active Directory schema changes are permanent, and cannot be undone (at least, not without rolling your entire domain, including every domain controller, back to before the change).
Here is my question: Is it "dangerous"?
Not really. The permanent changes that cannot be undone are simply extra attributes and object types that are available in the directory, but won't hurt anything if they're not being actively used.
Many domains out in the real world are in this exact situation, as any organization that has ever used Exchange in their domain has some version or other of the schema extensions installed regardless of whether they're actively using Exchange today.
This isn't to say you shouldn't continue to be suspicious of applications wanting schema extensions - but Microsoft's extensions to Microsoft's LDAP schema are pretty likely to be safe. They're sometimes even useful if you're not planning on using Exchange at all - see this question, for example.
Everything other than the schema changes can essentially be blown away if you decide not to use Exchange.