That's exactly the case. But I've talked about it before. It depends what you go in hoping to hear/see. I have different hopes than most. The bands worth seeing are the ones who elevate beyond what is available on the album. Improvisation isn't a requirement (though obviously it helps), and certainly stagenastics aren't either, but the band has to sell their performance, in some form or another. Many bands are just going through the motions in live performances, or for some reason their performance just doesn't work for me.
Tyr is a band that works for me live. Eluveite as well. Hell, Blind Guardian works great for me live. So does Kataklysm. But I'd say that if I enjoy a band live, there's about a 30% chance I'll enjoy their live show. Partly, its because I really do think about 'was this worth my time and money?'
Unfortunately, I haven't seen Opeth live.