If you just don't want to buy a seperate moisturizer for night time then it sounds fine. My derm once told me to use my Olay Complete in the AM and before bed, and it had SPF 15 in it.
If you are using it for the SPF, it might be a little unneccessary unless you live in a region that gets 24 hour sunlight in the summer (IE: Alaska)
Well, if it's your daily moisturizer and it happens to have SPF 15, then I don't see how it's going to hurt. If you're buying separate moisturizers, then maybe don't bother with one containing SPF for at night.
good question!! i never use spf in my moisterizer cause my foundation has it and the less i have to put on my face the better... i have sensitive skin though