The reason you use a hydrometer is, as you suggest, to take the original gravity and the final gravity. Bubbles are irrelevant and should not be relied upon to tell when fermentation is complete.
This is true for any type of brewing - why you singled out kits is a mystery.
A KK brew that uses dextrose will have a different OG and FG to one that uses all malt or a proprietary brewing sugar.
Any yeast can stall or even not begin in the first place. How do you tell if a brew has started fermenting? Krausen, condensation and above all - dropping gravity are the best signs.
Many people buy a fermenter, get a can from the supermarket and brew away so the kits coming with plastic bottles doesn't save people from exploding glass if they don't know what they are doing and if they are using glass. I inherited my dad's old gear and started with glass bttles (still use them). I'm not alone.
Finally - regardless of what vessel you use for your finished beer - plastic bottles, glass bottles or kegs, why wouldn't you want your beer to have finished fermentation before you have put it in there? There are all sorts of reasons besides kaboom that mean allowing ferment to finish is a good idea. Kaboom is a particularly good one though - especially for new brewers.
A hydrometer costs around 15 dollars and can save anything from headaches to eye injuries if you learn to use it properly.
You don't bottle because the hydrometer says so. You bottle because the hydrometer gives you consistent readings within a range that you would reasonably expect represents finished beer. It's an instrument that gives you a measurement. How you interpret that measurement is up to you but without it, there is no interpretation. Just because you misused a hydrometer in your early brewing days, doesn't make the hydrometer a useless instrument.
