Because getting trapped in our own zone for shifts at a time doesn't always reflect the line that starts it. The fourth line might survive it long enough to chip it down the ice and get off for a change, but then by the time our next line can get over the board, here comes the other team again putting us back on our heels. It wears our guys down, and with the other team controlling possession and zone time, raises the likelihood that they'll put a shot on net that'll squeak through.
Also, it shows how little we trust our top guys. How many cups have been won by teams whose top guys couldn't go out and beat the other team's top guys? In other words, how many teams relied on a pure shut down line to get better match-ups for their goal scorers? The last team that I can think of that really employed that strategy, and won a cup, was Anaheim. But they had Pronger and Neidermeyer back there to eat up 50+ minutes a night while Pahlsson and Moen (and whoever the third guy was, Rob Neidermeyer, maybe?) clawed their way to a stalemate every shift.
If the Glendening line could more consistently tilt the ice, it'd be less of a problem. I still don't think they should be getting more ice than the Sheahan line, though, for example. Or a Datsyuk line, assuming Datsyuk comes back to center a separate line.
Also, whatever we're doing isn't really working as well for the team anymore, either. The entire month of November has seen our goals against increase. Out of ten games this month, we've given up less than three goals just three times. We can try to sort out which line is at fault for it, or what not, but it might just be about how we deploy our lines.