These people go around testing waters, but usually end up pulling their foot away from the water, as they do not want to get soaked into another person's life. They enjoy tasting, but they never complete a full meal. They have their own way of finishing things off which to most people would call quitting; when things get tough, they walk out, BUT they leave one foot in, just to make sure that you will always be available if they need you... you know...like a spare wheel; not good enough to keep the car rolling for long, but when you're the last hope, they hang on to you for emergencies.
This is a cliche saying, but I love it and so I do not mind being cliche in this post: "You treated me like an option so I left you like a choice." In relationships and friendships, being an option and usually the last option is not a vacancy that any of us fancy filling, but we do it. We put false hope in the people standing at our doors and pray for the day when they will walk in completely then we blame ourselves when they walk out completely for letting them linger at our doors for too long.
Testing the waters is good practice, for who would plunge into the unknown so readily only to find that the water was too deep or too shallow for them. Giving people time to linger at our doors and test our waters is a great step for the beginning of a relationship, but it is not a permanent parking stop. There is a time limit and you must enforce laws on your properties and forbid any trespassing to strangers. Give them some time to linger and think, but after their time is over and they still have not come in, shut the door before all the warmness leaks out and you are left cold and alone.