... I assume once you cut the rest of the strip is pretty much useless is that right?
Nope, you
can use the rest of the strip. The design of these strips make them extremely useful. Assuming you got one of the standard strips, every (3) lights is technically a separate entity if you cut the strip properly. You could, in theory, create dozens of little 3-light strips from a larger 15m strip if you want. As long as you cut on the proper line, you can keep using the strip as much as you want. For one of my marquees, I measured off 24" or so, cut at the nearest cut point, wired it up to the 12V on the cabinet power supply, and boom lights. I then went on to use the remainder of the strip in other places on other cabinets without issue.
One small additional piece of advice. Test the lighting out, see how much you need, and what angle works best. If you got "bright white" rather than "cool white," you may find it too bright if aimed directly at the marquee (depends on your marquee material, preferences, etc). Just try different things, different lengths of strip, or even multiple strips inside the marquee if you want. I've seen people mount them inside of aluminum foil "boxes" to disperse light, aim them backwards to reduce brightness, even add dimmers and control features. You'll have plenty of options. Personally, I would wire it direct to a 12V source (either existing arcade power supply or 12V wire on your PC power supply, but adapters work okay too if you're setting up a power strip in the cabinet.
Good luck!