It is the 19th of April 2017 and there are still people discussing how useful an assumption of climate equilibrium happens to be. Many of the pro equilibrium crew are obviously liberal arts majors.
Equilibrium is very useful if the initial condition assume to be at equilibrium is valid to some degree of precision that is much more accurate than the change one is trying to measure.
At the Top of the Atmosphere you have Ein ~ 240 Wm-2 and Eout ~ 240 Wm-2 if you average both over a reasonable period of time. Both Ein and Eout vary by close to 5% over the course of a year and the change being made will be about 1% of the potential energy at the surface.
The surface energy is roughly 390 Wm-2 based on an average temperature of 288K degrees plus about 88 Wm-2 of latent energy on average plus around 20Wm-2 of energy related to convection. There is another 4 Wm-2 or so of mechanical energy related to ocean and atmospheric currents and several small sources of energy. Just using the bigger guys, you have 390 + 88 + 20 = 498 Wm-2 that will produce the ~240 Wm-2 at the top of the atmosphere. The uncertainty in that 498 Wm-2 is about +/- 17 Wm-2 based on Stephens et al 2015 I believe.
So the answer to is the equilibrium assumption useful question, depends on how many knock on assumptions are based on the initial assumption and how much error there can be because of the initial assumption multiplied by the sensitivity of knock on assumptions to that error.
The simple answer is still maybe it is valid, maybe it isn't. Time will tell.