I have to disagree that the two are mutually exclusive, and that a leader is good and a manager is bad. I mean, Jim Jones was a very effective leader, instilling confidence and fierce loaylty in his followers. He led by example, people were willing to follow him...and he led them all to certain death. Is that good?
And I have worked with many managers who juggle a thousand balls at once and handle all of the important details to ensure that their employees have the resources they need to be successful, get the proper training, get paid, have health insurance, etc etc etc. Are they out there in the field? No. But would a salesperson be effective without a cell phone, literature. training, leads, flight arrangements, and so on? Would a soldier be effective without their gun, food, Jeep/Hummer, maps, ammo, plans, communications, etc?
So my definition of a Leader is someone who is out front, metaphoricaly and/or literally. They are showing the way, setting an example or establishing a path for others to follow. This can be good or this can be bad depending on how effective they are and where they are going. The key is, you are only a leader if people are willing to follow.
My definition of a Manager is someone who keeps things going. A good manager finds process improvements, greater efficiencies and improved logistics in what they manage, and is someone people turn to for guidance and input. A bad manager simply oversees a process, perhaps one that is largely automatic or redundant, shuffling papers and cashing paychecks.
I think that a manager can also be a leader because they are able to inspire and lead their team to success. I don't think Leaders make good managers however, in fact, the most successful leaders in history probably had good managers behind them! But without someone blazing the way, there would be nothing to manage.