WhatsApp is not perfect and certainly its code not being available for inspection is one of its flaws. However, it did bring security to the masses. I mean I am pretty sure the security it offers is enough for 95% of people. I would not use it for sending documents stolen from NSA, but for the rest of cases - it gets you covered. Security very often need to be balanced with convenience - with WhatsApp you get immense boost of protection without sacrificing much convenience. One could argue, that you could get better security with Signal - true, but first you'd need to convince all your family and friends to install it.