The thing is that VSBRs on pierce mode sacrifice area damage in order to actually punch through barriers, armor and beam shields. It's a tradeoff, the TBR is massively powerful and capable of dealing much more damage than a VSBR, no doubt, but the VSBR's pierce mode is designed specifically to make beams thinner, more concentrated and faster entirely to punch through defenses, even if it makes them deal less structural damage than a standard beam shot, which they can also fire I might add.