Lots of times, CD purchases are driven by bands you may be going to see live. Lots of factors.
Good point. I can imagine a kid fairly fresh into metal going to see some band like Nevermore that his older cousin turned him onto. Since his slate is pretty clean, when he sees a band, like, I dunno, Blackguard, he says "whoa! Synchronized headbanging is the most awesome thing I've ever seen!" He immediately becomes a fan and buys a shirt (and then goes home and downloads their album...this is a kid, remember!)
It matters not a whit to that kid how many tours Blackguard had been on in the past, because he wasn't at any of those shows*. The important thing is that Blackguard connected with a new fan at *that* show, and the more shows they play, the more chances they have to make such a connection.
The amount of music out there far exceeds anyone's ability to be even aware of it all, much less listen to it. Thus, to get people to actually hear your music, bands need to use a variety of tricks to cajole people into listening, and out-compete the thousands of other bands vying for that same prize. One trick is to write songs that people tell their friends about. Another is to spend a lot of money on straight advertising. For bands without those abilities or resources, a third trick is to get attached to tours. If there are two bands with the exact same songs, I hope we can all agree that the band that tours their ass off is going to end up with more fans than the band that stays at home.
So back to Stratovarius. You could probably find plenty of people who would tell you that Stratovarius peaked creatively and perfected their formula with 'Visions' in 1998, and should have just called it quits after that, instead of crapping out stale and uninspired rehashes. But if they had done that, you and others in this thread would have never become fans. Sure, nothing would have stopped you from going back and picking up 'Episode' or 'Visions' in 2001 (just as Blackguard could have done one tour and then relied on people to seek them out), but clearly that's not how people hear music that's new to them. It generally has to be shoved in front of them somehow, which Stratovarius did for you by releasing a "new" album of leftovers, a pure promotion/cash-in built of songs not good enough for their normal albums.
Hopefully that helps you see the world through the perspective of someone whose eyes aren't your own. You think Blackguard shoving themselves in front of people is ineffective and perhaps even damaging, because Blackguard's shoving doesn't work on *you*. But Stratovarius's shoving worked on you. And you just said "Lots of times, CD purchases are driven by bands you may be going to see live", so you clearly understand that there is some relationship between live shoving and CD sales, so you're only a synapse-connection away from agreeing that Blackguard's constant touring can be good for them.
* some stats (as always!) to back up my postulation that Blackguard's "overexposure" is much more a figment of your imagination than reality: I go to far more shows than most people, 135 over the last 5 years, and the total number of times I've seen Blackguard, Swashbuckle, Powerglove, Into Eternity, or any of the other road-dog bands mentioned in this thread is...3. One each for Into Eternity, Swashbuckle, and Blackguard. And I believe you've said you haven't even seen Blackguard a single time? So how do you conclude from that that Blackguard is actually "overexposed"?
Neil