I mean, if you ran a 100W laptop off a Corsair CX430 that was also powering a computer, the PSU would just shut down. I mean it can only do 20A@+5V, which is 100W; an average computer pulls 30-50W on that rail, so 130-150W exceeds the PSU's design parameters. Not to mention if you're doing this at idle the +12V load will be very low, so you get a crossloading situation. The +5V voltage goes way low, and the +12V voltage goes way high. There's potential for component damage there, and system instability. Low +5V can cause a hard drive head crash.
Most PSUs in pre-built systems are even less capable than the CX430 in this regard.
They could redesign these PSUs to have more +5V capacity, but that seems a step backward as we've been phasing these voltages out slowly since 2001.