Dynamic scoring of budget proposals has been on the Republican wish list for decades. They have always been frustrated that tax cut proposals look like such budget losers with static scoring. In their supply-side bones, Republicans know that tax cuts will stimulate economic activity and thus increase future tax revenues. Taking this second order effect into account is what they mean by dynamic scoring (see: Laffer Curve).
I have some sympathy for this argument, but in making it Republicans are falling for the "this will work great when our guys are in charge" fallacy (I need to find a name for that). Democrats fall into this all the time, expanding government power only to be shocked at what their political enemies do with this power once in charge.
Because it is pretty clear what dynamic scoring will mean in a Democratic Congress. Remember that stimulus bill? Democrats all thought that expanded the economy, so its costs would, by their Keynesian assumptions, appear much lower under dynamic scoring. The Left thinks the auto bailout was stimulative. They even think that Obamacare was stimulative. Do you really want some BS Keynesian fudge factor obscuring the true cost of such proposals in the future?
Related: Greg Mankiw discusses why, if I read him right, dynamic scoring is impossible to do correctly