You can't use them for more than 5 variables efficiently and no "real life" circuit has only 5 variables (except for the simplest of them). So what's the point of them? They seem pretty useless for actually engineering work.
In today's world with many decent choices of hardware design suites with HDLs (Hardware Description Languages) and Digital Circuit Optimization and Minimization there is no real need for small topology logic design as offered by Karnaugh maps. On the other hand there are two pretty useful areas where they come into play.
A) One area is in the training and education field. Karnaugh maps are an excellent visual tool to help students learn about the ideas of logic minimization and essential prime implicants.
B) Another area where I've found them useful is in embedded programming. Once in a while when programming an algorithm or a software state machine a quick Karnaugh map can be used to simplify the conditional expression used in the code. Four, five or even six variables is often just the right fit for a programming logical expression.