First off, a defense of messages like that one. Keep in mind that the welcome isn't for us, it's for the new user. Even if the same welcome message has been posted plenty of times and you're tired of it, it's the first time they've seen it - that's why they're being welcomed! So it doesn't seem nearly so robotic to them as to you.
Also, while making them feel welcomed is half the goal, letting them know about the ground rules is really important too. A lot of people expect discussion forums, and are really confused by the way StackExchange sites work the first time they visit. They post not-really-answers, they take downvotes personally, and so on. Pointing them to the tour/help at the first opportunity is an attempt to avoid this.
So in my mind, the canned messages are fine. I might tweak the one you quoted a bit, but there's nothing wrong with something along those lines. If I wanted to go beyond that, I'd simply to throw in a specific comment as well - something very simple that I liked about their post, or a suggestion for improving it. But that's not always too easy, and I don't think there's anything wrong with just using the template.
That said, if you want more variety, why not just welcome them yourself?
The moderators don't own the site. Their primary job is to do the things that no one else can. This early in a beta, there's a little more in that category since there are fewer high-reputation users. But welcoming a new user? All you need to be able to do is comment.
So if you want to contribute some variety, go for it. A variety of users doing the welcoming is perhaps even more meaningful than a variety of messages: it demonstrates that the site is a community site, not one kept carefully under the control of a few moderators. Just keep in mind why you're doing it: to make them feel welcome, yes, but also to help them learn how the site works so they can make good contributions.