A dealership can do the work, but you will pay more. They are also trained to sell you extra repairs.
As someone who actually works in a car dealership, I call quite frankly...
ahem on this one.
Not all plans or companies will honour your extra warranty if certain maintenance repairs and intervals are not done at a dealership. Sure, we have one douchebag of a service advisor at our place, but he is definately in the minority.
Furthermore, you get what you pay for. This is not to say you are guaranteed a great tech at every dealership, but our techs who started out at the local community college, eventually doing their apprenticeship at our dealership, focused in school on the company they currently work for. This isn't true with all of the older techs, but the ones who've arrived in the last decade? Yes. Even the ones who began ended their apprenticeships/work placements when I started 4 years ago, really had nose-to-the-bricks in the company they currently work for. It's not to say they couldn't work on a non-company vehicle (we do on occasion) but there's a level of expertise there that you may not nessecarily get if the tech is focusing on every kind and make of vehicle.
We have taxicabs that come in for the occasional recall, who take their vehicles only to local mechanics shop, and they're in terrible condition. And before I get the standard 'Well, they're always on the road!' My father, who takes his vehicle to the dealership (employee discount) is still on the road with the same truck for work at 400 000 clicks on an '03.
Other than the clutch on my dad's Midlife Crisis Mobile, we've had no issues with no dealership. And that's more of a manufacturing issue (it's happened with the gentleman who has the '07, and another who also has an '08) than an issue that arose out of shoddy care at the dealership or crappy maintenance work.