Were Ritchie and Isbister really necesary?

With the recent signings of Byron Ritchie and Brad Isbister the Canucks now have the dubious honour of having one of the NHL's deepest 4th lines. Assuming Linden is re-signed here are all the players that could concievably play on the Canucks' 4th line:

Ryan Shannon, Trevor Linden, Brad Isbister, Byron Ritchie, Alex Burrows, Jeff Cowan.

Now I'm not really confortable with any of these guys playing anywhere except the fourth line for long periods, but even if we get desperate and end up with someone like Ritche or Cowan filling out the third line with Cooke and Kesler that still means two of these six guys won't be on the team to start the year. Which brings me to the question: Why bother with Ritche and Isbister? Why not spend the $1 million or so you spent on them combined on an established third-line grinder (Re-Sign Bulis maybe?) and take your chances with Linden Shannon and Cowan as your fourth line with Burrows and Rypien as injury replacements. I'm not against depth at any part of your team but it seems that the 4th line isn't really the spot you'd most like to have it, especially when you're still missing a second and third liner.