I will dissent from those who say not to use tube glue at all. I use both tube glue and liquid glue, in this case, Testor's in the orange-and-white tube, and Plastruct's Weldene. Both have their application, and advantages and disadvantages, depending on the situation.
I have not found tube glue to tend to string, based on its age, but rather, based on the quantity that you squeeze out of the tube, and the method that you use to apply it. Try not to squeeze too much out at one time, for one thing. I use toothpicks and straight pins as applicators. I've found that I can minimize the string that tends to develop by applying the glue to the piece, then to start pulling the applicator away enough to let the string start, but then to let the string fall back in the mass on its own.
Strings are the big disadvantage with tube glue, in my opinion; the main advantage in tube glue is its longer setting time, which allows me to adjust a piece before setting it aside to cure. An example where I would use tube glue over liquid glue is in attaching a piece of stretched sprue to an aircraft as an aerial. I can apply a dot of tube glue to the attachment points, then attach the sprue, and adjust it, before the glue sets up. The liquid cement tends to melt the sprue right away, in my experience.
Liquid glue has a big advantage in mating long joins or seams together, like two halves of a fuselage or top and bottoms of a wing. It's very easy to flow the glue along the inside of the seam, and it lends itself much better to the "squeeze" method of pushing the melted plastic out along the seam, which fills the seam.
This is my opinion, based on my own experience, of course. I'm just saying, I find both types of glue have uses and have a place in my tool kit. Same goes for CA glue and two-part epoxy glue.
Hope that helps!
Brad