In the old days...
Well, about 18 years ago there basically was almost nothing on bikes except 2-piece bars. I remember the Redline bars that came out early on with a V shaped 4-piece bar, but it was really the Haro Kneesaver bars that came out around 1987/88 that really changed the shape of bars.
From the late 80's though the 90's all bars went to a 4+ piece design. The added strength that multiple bar handlebars (4, 6, 8 piece designs) was quickly proven and there was an added 'cool' factor to having bars that were a little different than Wal-Mart bike bars.
In recent years: Bars used to break because they were many INCHES wider than the bars on most riders bikes today. Now riders run narrow bars which are much less likely to break in a 2-piece design. So, 2-piece bars can once again be made and they are MUCH less expensive to produce.
Weight is not much (or any) better with a 2-piece design vs. other designs. Strength is definitely not as good with a 2-piece design - the bend in the tube is the weak spot. But, because there isn't as much stress on the bend because the bar is narrower than it used to be, the bar is strong enough to last years.
4, 6, and 8 piece bars are still used by some street/dirt/vert guys, but mostly it is flatlanders using them. I have run 8-piece bars for 10+ years now and would be hard pressed to use something else. Flatlanders still need the extra strength when they have a crossbar that is really low. It increases the stress on the bars a ton when that crossbar is dropped several inches below the rise of the bar (see pic below). The added stress needs to be made up for by using separate straight tubes, then heat treating the steel to make it stronger. The company that revolutionized handlebar quality was Graveyard Products which was the first to really push a high quality, hand built, 8-piece design. That was over ten years ago and as far as I know, every 8-piece bar has roots from Graveyard.
Check the weights of 2-piece vs. 6/8-piece bars and you will find that most weights are comparable. But, the 6 & 8 piece designs are definitely for flatland with a very low crossbar. This, in the end, makes those bars unsuitable for street/dirt/vert for long periods. A good street 4-piece or 6-piece design will be much stronger than pretty much any 2-piece design. But, they would cost significantly more... and not many guys want to dish out $100.00 for bars when they can get em for $30.00 instead.
NOTE LOW CROSS BAR ON THESE QUAMEN HANDLEBARS (Direct descendant of Graveyard bars)
