In most places, density is bad. In bangsar south, the jam is mainly caused by the offices (working crowd) and less by residences.
In any TOD, a good mix of a self-sustainable community (more office and residential) is important to fuel the vibrancy of the retail lots in Bangsar South.
Taking South view as an example, most of my prospects did not really need a car. They take public transport or walk to work. Another interesting thing is that, not necessarily all Bangsar south prospects work in bangsar south. They just like the convenience, and may work in KL, PJ, Bangsar.
My point is, with more units within the area is a concern for "competition of rental rates" within bangsar south and not simply because of density (unless the development has inadequate lifts). Density is a concern for inaccessible developments that u have no choice but to drive.
Note: I can't say for the others, but I rented my south view unit within 2.5 weeks of getting my reno done, based on a higher pricing (psf and absolute) than Southlink. Hence, I feel that southlink is pretty much a no brainer