These shortcuts make assumptions about which grenade you take and prime. Suppose there are multiple ones (like EMP grenades, gas grenades, regular grenades, dynamites, etc.) and they have been equipped at various locations on the soldier's body. Meridian's point was, I think, that implementing such shortcuts makes some assumptions and will in many situations (if there are many options to choose from) lead to unexpected results that players will be dissatisfied with.
Now, as a player, the only thing I would like to see more straightforward would be a way to eliminate the turn counter when priming the grenade. I have only seen it used once (in Stone Lake's XCF streams) and never used anything except 0 myself. IIRC BAI already does this.
As a player, when I'm using grenades extesively I usually preprime them when equipping the soldiers. Then using and throwing them is much faster and smoother (and you can throw 2 or 3 grenades per turn). But this is often only an applicable strategy in the early game. Later, when your soldiers improve you might switch away from these tactics. And don't want to preprime grenades later on because if the soldier gets knocked down, the grenades are going to go boom. So you'll also need to be careful with how you deploy your troops. Still, a valid strategy especially with sniper/spotter mods where you'll need to avoid shooting certain enemies.
