Yes... I agree. If bass is your main target with an occasional trout trip. A 6wt might be your best choice, especially if you're after larger bass. Myself, I mainly trout fish with only an occasional bass trip. So if I were going to choose the rod for myself then I would go with a 4 or 5. I have caught 4lb bass with 4 and 5wts and it works fine. However, as Cliff suggested, when the bass get over 4lbs you may need a little larger rod. A 6wt would likely be in order. Also, if you're going to be bass fishing with streamers and need to cover a lot of water you're probably going to want to make long casts. If that's the case a 6wt will serve you better.
Now... a 6wt is too large for fishing for trout at, say, Beaver's Bend State Park. I use a 2 or 3wt on that river. A 4wt would be fine and a 5wt would be OK. I use a 2, 3 or 4wt on 99% of the trout streams that I fish.
Ideally, two rods would be great. A 3 or 4wt for trout and a 6 or 8wt for bass. However, now we're talking twice the money. But with the price of quality rods dropping, its a little easier to afford a complete arsenal. It looks like the majority of the folks that responded to our previous rod poll owns over 5, and even over 10, fly rods. The multi-species issue is the main reason why.
Anyway... for only one rod to serve multi-purposes, you just need to decide exactly how much of what type of fishing that you'll be doing. You can find the right rod for you. Take your time and don't rush your decision.