Custom ROMs, yes we do have, but not as near as many that we could have if we had an unlocked bootloader.
It's the custom kernels/recoveries we cannot have.
But we have a recovery?
Yes but it's a 2nd-init recovery, meaning that it is not permanently installed. If you got stuck in a boot loop and CM10 was not installed and factory resetting doesn't work, you would have to unbrick to get out. Or if you get the security error because of deleting/flashing something you shouldn't have you would need to unbrick.
With an unlocked bootloader we can flash any recovery, kernel, ROM we want that's made for this device.
With a bypassed bootloader (with the help of loki), we can flash any recovery, kernel, ROM we want that's made for this device and patched with loki.
It's hard for devs to work on a device where the kernel cannot be touched. They have done very hard work to get around security errors from a locked up bootloader. It would have saved them a lot of time if the bootloader was unlocked. With that time saved, we would be able to see a lot more ROMs from devs.
And I mean ROMs built by source.
We would also be on CM10.1 by now if the bootloader were unlocked.