it all depends on what TYPE of AR you are building. If you are building a KISS/off-the-shelf/m4 clone then yes, it makes MUCH more sense to go buy that AR-15 complete.
In my case, it is not worth it for me to buy a complete AR, and then swap out 70% of the parts on it. I then have to sell all the take-off parts I wont need (handguard assemblies, chg handle, stock, pistol grip, flash hider, trigger, carry handle), and then have to buy all those replacement parts in order to have what I want.
ARs arent cheap, and the lessons i learned from the first 2 ive built meant that when it came time to build 3 and 4, i built them right the first time and wasnt swapping out parts on a weekly basis.
My ARs 1-4 (1&2 have been sold):