I’ve played in a lot of games where things could have been a lot easier if we’d only had a Cleric in the party. Or an archer. Or someone trained in Thievery. Or a controller. The point is that some obstacles are going to be easier if you have the right tools for the job. This is also true when it comes to PCs. The right mix of classes and races in any given party will provide you with a competitive edge that will make many tasks easier.
Many DMs design adventures knowing what tools, skills and abilities the PCs have and create challenges that their unique skill sets will be adequately suited to overcome. However, there are often just as many times where the DM simply needs to throw certain monsters or other obstacles at the PCs and if they don’t have the right tools for the job then things are going to be a lot more difficult. This is especially true of you are playing form a printed adventure like those found in Dungeon magazine. The key to overcoming this issue is to try to ensure that the party is made up of the PCs most suited for the job in front of them; a task that’s easier said than done.