I'm late to the party, but you all have it pretty well covered.
As with most ports and inland waterways throughout the world, ships on the Great Lakes are required to have pilots aboard while underway. Foreign ships coming in for shorter stays just pick up pilots along the way as they pass through the different districts. Vessels native to the Lakes meet the requirement by employing deck officers that have pilotage endorsements.
Different vessels have different requirements. My ship and most like it are required to carry three pilots aboard. I have pilotage, and two of my mates have pilotage. We take the same tests and get the same endorsements as the pilots. My third mate hasn't finished getting his yet, but he's working on it. He will get a small bump in pay when he gets it, but the main impetus is that the company requires mates to have pilotage as a condition of employment. They'll let you work here for a year without it, but they won't wait forever.
We don't make nearly as much as pilots do. In fact I typically make less than half what Great Lakes Pilot would make. I honestly don't have a good enough understanding of how pilot rates are set to be able to chime in on that topic, but I do know that the Coast Guard does have a say in how rates are set in this part of the world.
I think most deck officers put the pilot's gig on a pedestal as a sort of 'holy grail' of sailing jobs. There are plenty of days when climbing up that wobbly jacob's ladder from a pitching pilot boat seems like a great alternative to whatever nonsense I'm dealing with. Many captains I've sailed with have gone on to become pilots. They're paid better than just about anybody in the industry, you get to stay relatively close to home, and you don't have to deal with a lot of the ship's business that can make a captain's job unpleasant. You don't have to worry about maintaining the ship or it's endless binders of paperwork. You don't have to worry about complying with company policies, or dealing with crew conflicts, or crew changes, or the cook giving everyone diarrhea, or running out of milk, or toilet paper, or keeping inventory of how many of those stupid johnson rods you have in stock for the whats-a-ma-gadget in the lower windlass room.
The pilot job does have some downsides, though. You're effectively on call, and while you might have a fairly regular schedule during steady times, you're required to be available if needed. This means if there's a traffic jam, it doesn't matter if you've got a flight to Vegas booked. You're working as many hours as you legally can until that traffic jam is cleared. Also, on the Lakes, a pilot's area of coverage can be pretty big. It's not always like being a harbor pilot in Norfolk or San Francisco. You might be travelling hundreds of miles on a very regular basis, and will spend a lot of time on the road and in hotel rooms. It can be a lonely job. You're always the new guy aboard ship, and you don't have that camaraderie that you get by staying with the same crew all the time. There's also that wobbly jacob's ladder. The pilot transfer process is inherently dangerous, and people die every year doing it.
It's an exclusive club. There aren't all that many pilots out there, and it can be really hard to get your foot in the door. If you can though, you're at the top of the maritime food chain, and you can make a lot of loot. It's a high stress, high reward career.