Ok I see; yeah I misunderstood.
You are saying if you are part 107 and you fly commercial and you have a fleet of drones, why pay $5 for a bunch of drones you only plan to use recreationally? I'm not part 107 so I would think that if you are part 107 and you have a bunch of drones that you fly recreationally only, you would not try to add them (and pay for it) on your part 107 account but start a one-time $5 recreational account and add/register as many as you like especially if they are over 250g. I agree, I wouldn't start adding and paying for each of those either.
For the OP who is recreational and the suggestion was made to go ahead and register the drone anyway whether it needs to be registered or not (i.e. under 250g), I do the same and suggest the same....go ahead and add it for free and one day when you step up the battery or add strobes or whatever, you're all set if you have standard RID or you can go into the portal and link/switch on your external module. There's no downside to adding more drones to your recreational account.