Was killing Dumbledore "worth it"?

As we probably all know, Snape kills Dumbledore. Dumbledore ordered him to, because it was the only way to prove to Voldemort that Snape was loyal to him. But was it really worth it to lose one of the greatest wizards of all time, and in exchange get someone "on the inside"? What events would have gone differently if Snape was not trusted by Voldemort? And would a victory over Voldemort still have been possible?

I haven't read the last few books (I think I read GoF, and maybe OotP). So maybe there is more stuff in the books, but in the movies it doesn't really look like anything major is possible because of Dumbledore's death.