Why non repeated poles at imaginary axis makes LTI system marginally stable?
Complex conjugate poles on the \$j\omega\$ axis can also produce an unbounded output, just like a pole at \$s=0\$. It just depends on the input signal. If you excite a system with a single pole at \$s=0\$ with an impulse, the output is bounded (it's a step). If you excite if with a step, the output is unbounded (it's a ramp). If you excite a system with complex conjugate poles at \$\pm j\omega_0\$ with a sinusoidal input signal with frequency \$\omega_0\$, then you'll get a sinusoidal output signal with linearly increasing amplitude (a ramped sinusoid), i.e., an unbounded signal.