I bring Mr travelmad478 with me on a few business trips each year (mine are all international) and as you say, it makes a huge difference to my enjoyment of the trip. Having your SO there in the hotel at the end of the day, being able to eat dinner with the person that you really want to eat dinner with (as opposed to random business acquaintances or an empty chair), etc. is just a great thing when you can swing it.

It's also a good way to quietly demonstrate to your SO that business trips really aren't fun and glamorous, so they shouldn't get all jealous that you get to do them. There's nothing like watching me leave in a suit at 7 AM, return at 7 PM, and slog through dozens of e-mails until 10 to dispel that envy!