So should one be done and not both?
For my rifles, both are done, almost always, except in chassis rifles, which don't need pillars, but can still be bedded. Personally, I wouldn't ever put in pillars without bedding, but in synthetic stocks, as I mentioned above, a guy can bed without pillars.
They serve different purposes. Putting in pillars without bedding may just create a "bridge" out of the action, leaving two major pressure points, and not providing proper interface between the stock and the action. Bedding the action improves that interface, so if a stock is pillar blocked, it should be bedded too. Alternatively, again, if a guy isn't worried about warp/expansion/shrinkage/etc of their stock, they may only need to bed their rifle, and not need pillars.
I'm not a guy who buys the "shoot it to see if it needs it" manner of thinking. When done properly, blocking and bedding doesn't make rifles shoot worse, ever, and at worst, it makes the rifle easier to service. Typically, for factory rifles, it will make a notable improvement in precision.
So for MY rifles, block and bed, both, almost always. Bed always always.