Honestly, it's the most efficient payment method you could use. It's illogical NOT to get one.
When you buy one, you pay a deposit of 5 pounds. You get this back when you return it - so essentially it's free.
Then you put say, twenty pounds on the card - you can top up as you wish.
After that, ANY random bus, tube or train trip within London is half price. Why would you NOT use it?
If you use it a few times in one day, which it then works out as reaching the price of a day ticket, it stops charging you. Unless you go to a different zone of course, but even then if you hit the limit for say, two or three zones, it'll match that too.
Then if you're there for a week, you can buy, say, a zone 2-3 ticket and have unlimited travel in those zones for the week.
It's faster, cheaper and easier than paying for separate tickets.
For more information, check out the TFL website.