Unfortunately, you can't use multiple base days in one day. Disney has done the math too, and they know it would be cheaper for some people to purchase longer-stay tickets without the park hopper option that it would be to add the park hopper option to their current tickets. That's why they don't allow it.
Unless you are absolutely convinced you want to stay in one park every day, I would upgrade to a park hopper ticket that fits the length of your stay. It's always wonderful to have that option in case the park you first choose is really crowded or you just need a change of pace. I think having the park hopper is always worth the extra money!