Consider also how picky you are about the style of your eggs. I like my eggs rather well done. The French (as well as some Americans, of course) prefer theirs quite runny. In France I could only eat cooked-to-order eggs (which you probably won't get as part of a 10-euro breakfast) or hard-boiled (often available, but probably not always offered, and peeling them can take a surprising amount of time). My policy is always to opt out of hotel breakfasts wherever I can, because I can always find something a lot less expensive elsewhere. I am, however, prepared to settle for something continental style if I don't come across a carry-out option with something like mini-quiches or cheese sandwiches. I found mini-quiches widely available, usually for less than 3 euros.
If your party has some OJ rather than coffee/tea drinkers, consider that at some hotels the OJ is pretty poor. Not consuming a beverage makes a hotel breakfast a less-good deal.
I got stuck in several places that mandated breakfast on my most recent trip to France and England. A few of them had serving schedules that I found terribly early--ending breakfast at 8 or 8:30 AM. Definitely check to be sure that you won't be so constrained unless you know that all in your group will be rising early every day.
Otherwise, I agree that the convenience of a hotel breakfast is worth something, and 10 euros is not a bad financial deal if you are big-breakfast eaters and would otherwise be looking for an eggs-and-meat breakfast elsewhere.
Edited to add: If quality is a major focus, I suggest reading through the reviews on booking.com and trip advisor. Some of them will mention the breakfast, and you may be able to get a good sense of how it will appeal to you.
One other thing I should have mentioned is that buffet breakfasts can be very picked-over if you arrive just 15 or 20 minutes before closing time. A good hotel will keep the offerings well-stocked, but I walked into one that had two lone bagels on the bread counter and a flock of empty fruit bowls. It wasn't until the next morning that I discovered the breakfast was pretty decent, but you had to wake up early to get it.