Agreed, you can definitely catch fish without a depthfinder, but it's a heck of a lot easier with one.  What it mostly does is allow you to stay in the productive fishing zone for whatever you are targeting for a MUCH higher percentage of the time.  Also eliminates a lot of snags which maximizes fishing time and minimizes frustration.  It also allows you to find those highly productive spots like small fingers and slots around the edges of reefs that often attract and hold the most fish. 

Plus it's just plain fun to see what's down there.  MT is right that shorelines often give good clues to what you'll find underwater but quite often the shoreline tells baldface lies.  There are many areas where the shoreline looks like it gently slopes into deeper water but there is actually an underwater cliff just offshore. Or the opposite, where the shoreline is a cliff, but the base of the cliff is a mere 10' underwater.  You'll find underwater reef systems and sunken islands that never break the surface at all and are thus VERY lightly fished, often holding some of the biggest fish of your trip.

What it comes down to is this:  If successful fishing is a big factor in your enjoyment of a trip, it's the most useful piece of equipment you can bring save for your rod, reel, and a few lures.  And it pays to work on your setup so it's easy to store and deploy it before and after portages.  If it's a PITA to set it up and put it away, you're less likely to use it on travel days.