I'd like to know the answer to that as well. Once the phone is rooted, why would one need an unlocked bootloader. I can do anything I want with my rooted Incredible - at least until tomorrow, when I hope to pick up a Maxx :icon_ lala:....
Rooting without an unlocked bootloader means you have to cheat the system to gain root. An unlocked bootloader says, "OK, I'm completely yours. Do with me what you will!!" And you can!! When you have to exploit root, there could be things that go wrong with the system. When root is there and staring at you in the face, you don't have that many chances at errors! Just bad apps (if the app writer didn't know what they're doing) can take you down.
For me, rooting my Bionic was a way to use Titanium back up to freeze apps that sucked the life out of the battery. It also offered me a way to use wifi tether here in the United States like it is used in South Korea. I.e., the US is one of few countries that get's away with charging a subscriber another $30 to attach a pc via wifi tether.
I'm really not interested in overclocking or using other roms. I guess I left my pocket protector and my propeller hat somewhere. It would be nice of Moto to offer us an unlocked bootloader but it isn't going to happen. So if you insist on taking full control of your phone, perhaps buying a phone that is unlocked is a far less risky solution. I just wanted to be able to freeze bloatware and use my tether app without additional charges. So there you have it - a reason to root.