Like many tomato growers, I like the basil/tomato combination from a culinary perspective, but there's an additional reason to grow basil near your tomatoes.
Basil is even more tender than tomatoes.The leaves start to drop weeks before the tomatoes are harmed by the cold.
When the leaves near the top start to turn brown, or the stems look hideous like the one to the right, I know it will soon be cold enough to bring in the last of the green tomatoes.
Here is what a tomato plant looks like when it is first touched by a hint of frost, but hasn't gotten cold enough to succumb. The leaves are curling, the fruit start to go translucent. After a real frost, the fruit will freeze, and when it thaws its a mushy, blackening mess. Before that happens, it's time to bring in all the usable tomatoes, ripe or not.
I sorted the tomatoes.
Those with cracked skin, minor blemishes, or very wide stem ends (like the red tomato at the top) will not store well. They are too attractive to fungus and fruitflies.
Very green ones, or ones that have not reached full size, are unlikely to ripen. They work for green tomato recipes (fried, chutney, pickles, salsa) but I won't try to store them.
As Shakespeare says "ripeness is all". I think that over and over to myself as I harvest, and it makes me sad. It's not an inherently sad phrase - it's what you make of it. But right now, I think about being harvested too soon, or being past one's prime.
Fall is that kind of season. I try instead to think "season of mists and mellow fruitfulness". That sounds much better.
I make the real greenies and a few of the redder ones into green tomato salsa. It's tangy and sour when it's first made, but mellows over a few days and is delicious with corn tortillas and goat cheese.