A basic consideration for most people who embark on an adventure vacation is time versus money. If you have time on your hands and have basic skills in dealing with sports and the outdoors, then planning your own trip can be fun. On the other hand, making one phone call and writing one check makes a lot more sense if you don't have a lot of time and lack the background to safely get you where you want to go.
Transportation & Equipment--In general, the more remote the destination, the more you should consider an outfitter. In many parts of Canada, simply getting to the area where your trip begins requires a great deal of planning. Frequently, outfitters will have their own airplanes or boats or work in conjunction with someone who does. These transportation costs are usually included in the price of an excursion and are usually cheaper than the same flight or boat trip on a chartered basis.
The same rule applies to equipment rental. Getting your raft or canoe to an out-of-the-way lake can be an adventure in itself. But hire an outfitter and they'll take care of the hassle.
Another option is to use an outfitter to "package" your trip. Some outfitters offer their services to organize air charters and provide equipment for a fee but leave you to mastermind the trip.
Safety--Much of Canada is remote and given to weather extremes. What might be considered a casual camping trip or boating excursion in more populated or temperate areas can become life-threatening in the Canadian backcountry--which often starts right at the edge of town. Almost all outfitters are certified as first-aid providers, and most carry two-way radios in case there's a need to call for help. Local outfitters also know the particular hazards of the areas where they lead trips. In some areas, like the Arctic, where hazards range from freakish weather to ice-floe movements and polar bears, outfitters are nearly mandatory.
Other People--Most outfitters will lead groups on excursions only after signing up a minimum number of participants. This is usually a financial consideration for the outfitter, but for participants this can be both good and bad news. Traveling with the right people can add to the trip's enjoyment, but the wrong companions can lead to exasperation and disappointment. If you're sensitive to other peoples' idiosyncrasies, ask the potential outfitter specific questions regarding who else is going on the trip.