If you're patching low pressure ballon tires, or mountain bike tires, you might be able to get away with using rubber cement. If you're working on higher pressure tires, it'll fail, usually at an inconvenient or dangerous time. The patch and vulcanizing fluid are a system. The patch, on the surface that faces the tube, contains unvulcanized rubber and a small quantity of a vulcanizing accelerator. The vulcaninzing fluid contains a solvent (so you can spread it), unvulcanized rubber, and an activator for the accelerator on the patch. Put the two together, and you'll get a nicely cross-linked bond between the patch and the tube. Elmer's and the like contain only a solvent and vulcanized rubber. The accelerator on the patch is likely to stay unactivated, and you've just got sticky rubber holding the patch together. That can move, particularly under load, or when it gets warm. I've seen lots of good patches put on with ****ty rubber cement fail, even where I'm pretty sure the surface was prepped properly. I've seen lots fewer patches done with vulcanizing fluid fail, especially when the surface was prepped properly.