Worth it to just clear coat a marginally beat-up wood floor?
I am a firm believer in doing it right the first time. The most expensive part of this type of job is the urethane. Good quality oil based urethane can go $50 to $90 per gallon. you really need at least three coats to do the job properly, with a light sanding between coats. 

If you take the time to sand the floors now, get them in a good condition, balance the color if necessary, then put on 3 or 4 coats of oil based quality floor urethane, you will amazed how well they will hold up to kids and pets.  The other advantage is that spills, surface scratches and normal wear and tear can be easily repaired a few years from now with a light sanding and a couple fresh coats of urethane. 

The disadvantage of doing a quicky overcoat now over a damaged floor is that it will be much harder to sand, a lot messier later to remove all the urethane and surface damage on the wood. The job will take a lot longer and any $$$ you spent on urethane will be wasted.

If you do it right now, you will enjoy a nicer looking durable floor now and the freshening 3 to 5 years from now will be simple and fast.