If you buy Microsoft Points off Xbox Live Marketplace, you'll have to buy either 500 (£4.25), 1000 (£8.50), 2000 (£17), or 5000 (£42.50) at a time.
If you could buy 2100 Microsoft points off the Marketplace, you'd be paying £17.85, as Microsoft charge us 85p for every 100 points. Here on Play.com, you're saving only the 85p, but it adds up in the long run!
If you planned to spend £17 on Microsoft Points straight off the Marketplace, you'd get 100 less, and you can still get some decent stuff for 100 points! The best example would be a special Rock Band track!
If you're not in a hurry to get Microsoft Points, and you're happy to wait for three days at least, then by all means get this!