Totally agree, this is a feature, not a shortcoming. I use WhatsApp primarily because it is extremely fast when compared to SMS, iMessage is fast as well but not all of my contacts have iPhones.
If I want persistent group chat I'll use Hangouts or IRC/XMPP/Jabber etc. I have hundreds of options for that.
It's slower than WhatsApp in almost all aspects. I can send a WhatsApp message to someone who is 500 miles away on wifi and get a response from them in less than 2 seconds. It takes 2 more than 2 seconds for them to RECIEVE a message on hangouts. I've personally timed this and heard similar experiences with a few other people.
If I want persistent group chat I'll use Hangouts or IRC/XMPP/Jabber etc. I have hundreds of options for that.