Is RMS value of a pulse wave practically useful?
So my question is if we have a pulse wave does it make sense anymore
  to talk about its RMS value?


Yes, of course it does; the rms value of the pulse wave is the effective DC voltage across a resistor that gives the same average power.

Recall that the instantaneous power associated with a resistor is

$$p_R(t) = \dfrac{v^2_R(t)}{R} $$

The average power, over a period \$T\$, is then

$$p_{avg} = \dfrac{1}{T} \int_0^Tp_R(t)\,dt =\dfrac{1}{T} \int_0^T\dfrac{v^2_R(t)}{R}\,dt$$

Thus, the equivalent DC voltage that produces the same average power is

$$V_{eq} = \sqrt{p_{avg}\cdot R} = \sqrt{\dfrac{1}{T} \int_0^Tv^2_R(t)\,dt}$$

But, that last term is precisely the root of the mean of the square (rms) value of \$v_R(t)\$.

So, yes, it makes sense to talk about the rms value of a pulse waveform or any other voltage or current waveform for that matter.