i wouldn't say that someone like jan johnsten, justine suissa, sarah mclacheln, .. can't sing. it's much more the unbalanced vocal in the mix. if they would compress and add all those effects from the studio audio channel in the live performance it would sound much better.. but that's not that easy with all those complex and full sounding vocal trance tunes. you can't compare it to something like r&b, pop, .. which has almost nothing more than those vocals.

There's that, and there's also that a lot of these singers can sound fine on the vinyl because they can do many takes in the studio and then have their voice sifted through Antares Autotune. I'd wager to say that very few of them actually have perfect pitch, and hence can't find the right notes when they're nervous performing live.

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"When I read about the evils of drinking, I gave up reading."

Singing live and in studio are two completely different worlds.

Like Sean said, in studio you are in a controlled environment, where you can do several takes, and most of the time it's not even one complete take that is used, but the best bit of each part (that process is called comping). Just to point out, for instance on a rock album, it's VERY rare that a band just plays it in like that. Usually, each instrument is recorded apart, again different takes so you can make the perfect "collage".
And then there is the likes of pitch correction processors, like Autotune. Don't be fooled, these can be used live also, but in the studio you can tweak them much more precisely. If it doesn't work well, just change some settings, it's not like you only have one chance to nail it, unlike in live.

But it's easy to dismiss people that sing out of tune in live situations. Don't forget it's a very loud environment. Ever tried singing without hearing yourself? That's what stage monitoring is for, but that cannot always be optimal (although in ear monitoring is making it easier, but not all singers like that). The interaction with the room accoustics, even when using monitoring can throw you off very easily.
Saying there are no effects on a live vocal is wrong. The parameters are different, but usually there are effects on it (yes even reverb).
It comes down to experience. There are musicians (so not only singers) that are excellent on stage, but can't do anything right in studio (don't think that all the on stage band members of your favorite group are necessarily the same as those who did the recording in studio. Session musicians...). But the opposite is true also.

If you ever have the chance to be in a big hall, and speak/sing (even if you can't) in a mic, do it. You'll see how different it is from just doing the same in your living room.

But, talking about Sarah Mclachlan, she can sing allright. Even live.

justine suissa can't sing.. even after the full studio work out...

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insignificant cor member alliance

I remember a few years back in Ibiza when I saw a couple of live PAs that varied massively in quality.
The 1st was Amanda O'Riordan (Judge Jules wife) singing Angelic - Its my Turn. Words cannot describe how sh*t she was - horribly flat & no sense of timing with the track. Even Judge Jules looked like he was embarrased to be there.
Later on in the evening we had Marcella Wood (who is Matt Dareys Girlfiend) performing Matt Darey - Liberation and Beautifull. She basically kicked our arses - she sounded fantastic, clear & in-tune. They were both on the same PA & it just proved how much vocal fixing must have taken place on the Angelic track.

As to the original question about Sarah McLachlan - well considering she is an artist in her own right (she has stacks of albums of her own) & is probably used to performing live, I reckon she'll be great live.

Her & Jan Johnson are still me all time favourite trance/dance vocalists