Is it bad to have both a Canon & Nikon?

I'm looking to get a Canon camera (the t2i/550D) as my first DSLR. I found a great reputable store that sells it for a deal on ebay and I'm eager to get it.

However, we also have a Nikon (D300) in the family with 5 Nikon Nikkor lenses. Although most people consider it a disadvantage to have two brands in the family, I'm thinking it might be okay if not good. For example, if I need to take a macro shot I can simply use the whole camera body. Having the two largest camera brands opens you up to nearly the entire lens market. There's practically no lens that you can't purchase if you have both.

What do you think? What about adapters? I've read online that they sell adapters that allow you to use G-mount Nikkor lenses on a Canon EOS body. Are these okay? Do they reduce image quality/functionality?

Added (1). I will NOT buy additional lenses. I will not purchase an extra telephoto lens that is similar to a Nikon one I already have.

When I need to use those lenses I will simply use the entire Nikon body.

It really does not make a lot of sense. You will end up spending extra money on lenses.

This is a false economy. Stick with one or the other, perferably with the lenses you already own or have access to

I suggest you NOT go and get a canon if you already have a D300 and 5 lenses.

If you get a canon, you will end up re-buying lenses which can be disadvantageous in the long run.

There's no law against it. It does not go against any moral value or dogma. As far as I can tell it is not even illogical. You do present a good case. Having both leading brands indeed offers you the widest range of lenses in the market. Forget about adapters. You can find anything you need within those two brands.

Dear, it's your money. If you want to spend it, we can't stop you. I'm sure Nikon and Canon will thank you for this.

I might be confused, but clarify something for me. You said "we also have a Nikon (D300) in the family with 5 Nikon Nikkor lenses", but did not say that they were yours. To be frank, a Nikon D300 isn't something that I'd be passing around to different members of my family. Sharing lenses might be a different matter, but not by much.

You plan to get a Canon DSLR with ONE lens? That defeats the entire purpose of a DSLR.

Yes, it makes sense, only if you might justify getting a Pentax, Sony, or other cameras in the future because of a one-time, one-lens deal. Your family has a system. You're creating a one-lens annex.

My daughter wanted a DSLR, so I bought her a D50, giving her some of my other lenses. When she needs to, we share lenses back and forth. Her boyfriend has one of my older D70 cameras, and likewise shares optics. His sister recently got a D5000, and now we all can share (if necessary). You see, we're contributing to the mutual good.

By the way, you'd be better off just getting a D7000 or D90 body. But it's your (and your family's) call.