Your point is well taken.
In reality, most organizers of group cruises do so with the hope of recruiting enough participants to qualify for a complementary cabin and thus a free, or nearly free, cruise. If they don't get enough participants to qualify for a complementary cabin, they don't go and those who booked are on their own.
While you might expect this from people who informally organize occasional groups, it's also true of supposedly reputable travel agencies. By way of example, my parents booked a AAA group cruise several years ago that was supposed to have a representative from AAA onboard and a "meet the group" cocktail party a couple weeks before their departure. Only two other couples signed up for the group cruise, so there was no AAA representative and no cocktail party. Nonetheless they had a wonderful time.
If going with the group is what's important to you, go ahead and put down the deposit -- but inquire as to how many cabins the group will occupy before submitting your final payment. I understand that standard practice in the industry is to give one free cabin for the group's leaders or escorts for every ten paid cabins that the group will occupy.