As an investor, you can judge the current market size, the potential, as well where that fits into current trends when the entrepreneur's idea is present. Those are just some of the factors that are generally taken into account when making any investment.
If two otherwise equal entrepreneurs ask for money, and one has a product idea while the other does not, what is the difference in probability of business success between the two, assuming they both go on to secure the same amount of funding? If they are different, what is your reasoning for assigning different probabilities?