The main reason however is that, in presenting alternatives, you are shifting the decision-making power away from the person who is trained & experienced at making design decisions, and giving it to the very person who has hired you to help them because they don’t know what they are doing.
Let it be said that there are always a variety of ways to solve a problem. As a designer or developer, you should absolutely explore a variety of alternate solutions, but the result of your ideation sessions should be to come forward with a single recommendation. Even in an alternate approach to solving a problem, such as Agile development where you may be working more closely with the client more regularly, you are still working on just a single solution.
Ultimately, your informed recommendation is what the client needs. Presenting options dilutes the value of your work. Bring one strong, well thought-out solution to the table, and go from there.
For more on Paul Rand there is a good Miggs Burroughs interview with him from 1991 where he talks about design and branding.