Okay, sure, it can be good for the motivation and the spirit but you don't grow and improve on praise.
This is only true if the review is meant to be read by the author so that the author can learn and improve--which is not the purpose of many reviews. The purpose of many reviews is to help other potential readers decide if they want to give a story a shot--to add to that story's "visibility factor," in other words. And in that sense, a review that's very "gushy" might actually prove quite helpful. Though, of course, if that gushy review is full of lies and bullcrap, then it'll prove exactly the opposite of helpful, but yeah. That'd be a problem, too.
Also, please don't underestimate the value of boosting motivation. Sometimes, that's all a writer's got going for them, because everything else in their life is in shambles. Not that I have experience with that or anything.
What's more, praise is actually worth more when it's next to critique.
On principle, no, it isn't. That's actually just a trick. If you think about it, it's basically the same idea behind "negging." If a girl doesn't respond to your compliments, insult her instead. That'll get her attention and make her appreciate your compliments when you actually do give them.
Whether you believe that such a trick actually works or not is beside point. The fact is, it IS a trick.
If you want your praise to be worth more, it needs to be well thought out, too. And yes, more thorough and critical thinking often means that praise is accompanied by critique, but praise does not gain more value BECAUSE of said critiques. That's a classic case of correlation, not causation.
Maybe that's nitpicking on my part, but I think it's an important distinction for maintaining a critic's integrity and not getting caught up in a false dichotomy.