Disadvantages of building own components (such as SR latch)?
Really, the biggest reason is convenience. Lots and lots of components exist just for convenience. Why use a supervisor or reset monitor instead of a reference and a low voltage comparator with some hysteresis? Because reset monitors come in sot-23 packages. Simple as that. Hell, why do resistor arrays exist? No other reason than convenience (except for matched arrays, but that's a different story).

For prototyping and generally small series I'd recommend just using those convenience items. It produces cleaner schematics and makes your life a lot easier. Once you get into big production you can start looking into these kinds of things: thinking outside the box to save a couple pennies, consolidating parts, etc.