They actually do not appear that much. It’s just that when they do they stick out like a sore thumb, especially to anyone that finds them distasteful. I mean, what if one of his “Signature Weapons” was (speaking hypothetically) a sex toy? Every time they’d have a camera pan over his arsenal - there’d be that vile dildo, just sitting there. You’d then be reminded of it’s first instance in a Hitman game, probably shaking your head and/or cringing.
While I voted for more duckies, I only think there are a few missing that have special or different effects than mere explosions. Explosion (non-accident) kills are a very small percentage of kills when SA is the goal. Other than that they can be used to make proxy explosions for accident kills. Or to cause a panic, or open locked doors. But so can any other (non-duck) explosive.
Here’s the deal with the Duckers. Functionally speaking, they’re a deadly but legal item to hold.
Perhaps in a different universe, you would’ve had C4 Demolition blocks stuffed inside a teddy bear. 
Edit: I mean, we already have the Iconator, the Bear thing from the 7DS, and the waving robot cat.
And you can guarantee there’d be haters of explosive teddy bears. But! Here’s the best part… Nobody is forcing anyone else to use them. Need an explosion? Stick to your principles and bring in that illegal item that will compromise you the instant you take it out of your magic pockets within view of any NPCs.
One could also say they’re making the game easier by providing a legal item. But I’m reminded of a very integral part of hitman games. You can change disguises to enter areas where you would be deemed “illegal” (or trespassing). I don’t see disguising explosives as harmless ducks any different than the disguise system.
The ducks are fine. But I won’t blame anyone for being irritated at the numerous reskins. What else doesn’t have 20 reskins?