considering how intensely youtube pressures to you to SIGN UP SIGN UP LINK YOUR GOOGLE ACCOUNTS USE GOOGLE PLUS TODAY it's probably not going to be long before they do that too
Clearly you haven't been on youtube in a very long time, considering how they don't actually do that at all.
Even if the do pressure you, there is no reason to kill off 50% of their viewer base by requiring you to sign in.
cbut that aside signing up for nicovideo is literally just clicking two buttons, assuming you don't live under a rock lacking facebook/twitter/an NNID (you have an NNID if you are here, by the way)
I tried connecting an account and I still had to register with all of my information.
The only real benefit to connecting accounts is to save time by not typing in your username.