Why should it be restricted? Why can't that owl keep twittering around the big bad monster's head? If the big bad monster is sick of it, it knows what it needs to do to stop that freakin' owl.
And why can't a sorcerer help the cleric by applying pressure, offering an extra set of hands to prep and cut bandages?
Being able to help someone perform a task does not automatically mean you can do enough to help them be successful, nor that to be so helpful as to give Advantage.
As it stands now, a bat familiar, and sorcerer with -1 wis, and a Paladin with +8 Medicine skill are all allowed to 'help' the cleric with his medicine check... and they are all automatically effective, and they all give the same bonus (advantage)
But why not look for reasonable ways PCs can help each other rather than assume they can't?
Because they have to be helpful enough to make a difference (and a pretty big difference). Personally, I could 'help' someone hunt for food.... the best way would be to stay in camp. Or I could 'help' someone with their singing performance, and it might help, but doubtful it would be enough to make much of a difference.
I am thinking of something *like* this:
If you are proficient in the skill being used, you can automatically help.
If you are not proficient, you need to pass a DCX check
Not sure if X should be a set number, or DM adjudicated, or based in initial DC. (So to help with the clerics DC 20 medicine check, the sorcerer needs to pass a DC 15 medicine check... or something like that.)
Now, that kinda goes against the 5E 'streamline' philosophy... I would probably prefer/settle for stating that the DM should have say on what just won't work.... no your bat familiar can't 'help' you research local history...
I admit, I just have a particular disdain for the owl familiar flyby 'help' tactic....