What are the disadvantages of joining United's MileagePlus program?

There aren't any major disadvantages. In particular, there's no requirement to apply for the credit card.

There are some possible subtle disadvantages.

• Having a MileagePlus account might in the future sway you toward flying with United even when some other airline would otherwise be better (in terms of price, schedule, service, etc). That's the main reason United wants you to have one.

• You'll get more advertisements offering you the credit card, "special travel deals", etc, etc. You can probably opt out of most of this, but that in itself takes some time.

• It might be slightly easier for United to track you as a customer and develop a profile of your travel habits for their own marketing purposes.

• You may have to put some extra effort into periodically logging into your mileage account, checking the balance, etc. In particular, as Michael Hampton says, your account will expire if it's dormant for some period of time (I think it's about 18 months). This could happen if you don't fly with United very often. You can prevent this by donating some miles to charity, buying magazines, etc, but it takes some organization to keep track of it.

• You might get frustrated at some time in the future if the rewards of the program turn out to be less than you anticipated - maybe you collect a lot of miles and then find you can't use them for the flight you want, etc.

Everything in the other answer is correct. There are really not much issues with being a member of the program itself.

However if you are a member of another program in the Star Alliance, you will start fragmenting your points and it will take longer to accumulate a certain amount which also makes it more likely for your miles to expire.

Early on, I had made the mistake of registering for each airline that I flew, so United gave me Mileage Plus points, Air Canada Aeroplan points, etc. So each balance was not going up quickly and often an account or another's points expired. Now, I restrict myself to one program per alliance and so when I fly United, I ask to get Aeroplan points for the flight. Same for any other Star Alliance member. It would be good to compare the programs to see which is most beneficial to you before choosing one within an alliance. The benefits are not all the same and some work out better for short vs long flights.

If you fly United a lot, and you do sign up for a United credit card, you usually can get 30K miles as a bonus if you spend $1000 in the first 3 months. I know you said you wouldn't plan to use the card but it might be worth it to get the miles if you can reach the $1000.

The main reason I have a United credit card is you also get one free checked bag for up to 2 people (if you buy your tickets with the card and fly together.) That saves the two of us $100 round trip if we check bags which we usually do.

You also get to board the plane a little earlier, with all the other United card holders, in line 2.

There is no fee for the first year so you would could cancel it before the year is up and still get the free checked bags for a year.