Define need. You give up a large, growing, highly configurable, and easily extended amount of utilities by forsaking Plack and while I have never used hypnotoad except in playing around with mojo development it's difficult to imagine it's better than uWSGI in terms of stability and performance; judging by the docs, Mojo::Server::Hypnotoad, hypnotoad, it isn't even on the same planet for features. I looked around a little and couldn't find any performance or reliability metrics. As always, it's up to you and hypnotoad might be perfectly fine for what you want to do.
Update, obligatory overkill example: Re: Serving multiple Plack apps, Catalyst, Mojolicious, PHP, and CGI all served by a single server using Plack tools.