I mean isn't the best way to practice to actually play?
Most of the time this isn't true actually. Directed practice is the best way to actually practice. By that I mean, for example: if you want to get good at last hitting, you probably want to just do the "demo hero" mode and practice last hitting by yourself over and over until you get good enough that you next need to know how to deal with an enemy hero in your lane. Same thing for, say if you want to learn how to do Earth Spirit combos: it's best to actually load up a map by yourself, give yourself wtf mode etc and then just hammer out the combos over and over until they become second nature to you.
Unfortunately when the things you want to practice are less clean cut and more like "we want to get good at team fights" there's a lack of tools that will help with that, so playing real matches might be the best way now. I have no doubt that if the tools were available (e.g. having that old Replay Takeover feature that Source 1 had and 10 guys willing to try the same team fight out over and over), actually playing games is not a very effective way of practicing.
