If I were you, I'd let the cat onto the porch - if it's completely screened in, how much trouble could the cat possibly get into? After all, there are dangers everywhere, and we cannot protect our pets from everything.
My cat is 21 years old and still goes out onto the deck - it has a gate that is always closed, so she can't go out into the yard - she doesn't want to go out into the yard anyway. She also sometimes goes outside with my mom when my mom gardens. She is too old now, though, to actually go out into the yard and beyond on her own, but she did when she was younger.
She really enjoys suntanning on the deck, so I won't stop her, and my mom thinks the mental stimulation that comes from checking out stuff outside is good for her. When my cat was younger, she was very ambitious, and dominated the entire block - it was almost like she had her own empire and she was the ruler :-D I remember when I was a child, watching my cat interact with another cat in our front yard - I was amazed at how much like humans they were. I think this ambition is partly what has kept her alive for so long, but of course she's retired now.