[@SleepingSilence]

Well using your D&D example, under the perfect circumstances what you’ll be at is a spot to preform a coup de grace. It’ll be much different in a more active combat situation; you may attempt to stab them in the neck in the midst of heavy combat, but at best you’ll do a critical hit, or more likely just wound them. Worse, you might just miss because you were so focused on trying to slice their neck you weren’t prepared for their own ability to escape the situation.

Admittedly in the instance of Pokémon, what sort of attack you do can make things seem a bit weird when you’re wrapped up. But that’s also assuming you’re preforming a full body wrap; or that you have your opponent perfectly constricted. Like the assassin example, this is much harder to pull off against a prepared or at least reactive opponent. Take this [url=https://i.imgur.com/9ZPbdC0_d.jpg?maxwidth=640&shape=thumb&fidelity=medium]image[/url] of an arbok using wrap on an Aggron for example. Likely aware of eachothers presence the Arbok May have been fast enough to get a wrap onto Aggron, but it wasn’t as if he was completely paralyzed during the whole duration of the attack; obviously he would be resisting in some fashion as seen in the image. As such, Aggron still has many means to fight back. He could use metal claw to cut into the arbok, his tail is mostly free so he could use tail based attacks like Iron Tail, and it’s possible or that during the struggle the Aggron could strike the arbok with its head. With momentum and/or leverage an Aggron could even just slam the arbok around with attacks like double-edge or Heavy Slam, simply by using its own weight in the enduring struggle. 

The idea that you could get a perfect wrap onto your enemy to the point they can’t move their limbs to even struggle against you is like saying you, as an assassin, will stab someone so fast they can’t even try and parry you, At best it at least means that you’re trying to find an opprotunity or opening to effectively attack your enemy. At worse, you’re just hyping yourself up to godmode yourself into an automatic win with the excuse of “I’m fast so you can’t react”. 

That’s not also including that while you may be a seviper and thus don’t have an evolutionary growth, you are for all intents and purposes still physically immature for your species, close to a child like size rather than a more conventional adult body. In other words you’re probably not even big enough to wrap around things that aren’t already pretty small right now, like most bug  type Pokémon and baby level evolutions. You couldn’t, for example, wrap your whole body around a canine sized creature like an absol in any way that could be considered impending their ability to fight back at your current stage. At best you might be wrapped around their leg or possibly their neck.