I usually like, and prefer, when customers do not use POE injectors. This is, however, a personal preference. Here are my primary thoughts on POE injectors vs POE from the switch:
PRO #1: If you only have a few things to power (like APs, IP phones, etc), then POE injectors are good. The cost is low as opposed to a POE switch, so a good rule is that if you only have a few things to provide power to, go with POE injectors.
PRO #2: I mentioned this in #1, but if finances are the issue, POE injectors are good. They are cheap. Real cheap as opposed to a POE switch.
PRO #3: If the POE goes out in a POE switch, all POE has the chance of going out. If you have injectors, this isnt the case. If an injector goes out, it only affects one device.
PRO #4: If you do have to replace a POE injector, you dont have to bring any production down. You only replace the bad injector and you are back up and running without any production downtime anywhere else in the network.
CON #1: POE injectors take up more space.
CON #2: POE injectors take up more power slots for plug-ins.
CON #3: POE injectors require more patch cables. One for connection to the switch, and one for connection to the powered device. Thats twice what it takes from that of a switch.
CON #4: Most important to me, you can not reboot a powered device, if you need to, from a remote location. I would do this inside the switch config by disabling either the port or the power on the port. You would have to have someone physically find the POE injector you needed, and have them recycle power to it. WHAT A PAIN!!! if you are not physically onsite.
CON #5: They just look bad either hanging from the patch cable, suspended in the air below the switch in that sea of spaghetti of patch cables. I dont like it.
Mostly because of CON #4, I always prefer a POE switch. I guess that is because I have to do the troubleshooting of devices, and its just easier to disable a port and re-enable it again.
I posted this in my BrocadeFun site as well, since it is worth mentioning to both Cisco and Brocade folks alike.