I would like to add a short list of reasons why I stick with Marriott as well...
- Travel Packages represent a fantastic redemption value
- Premium Pounds & Extra Euros make stays quite affordable
- Marriott covers the full range from Basic to Luxury and respective price range
- Good coverage of the areas I usually travel

Downsides...
- no Upgrades to suites
- incosistency in complimentary weekend breakfast in the USA and no comp. Breakfast at Courtyards

So I think especially for the price sensitive traveller the advantages weigh stronger...

Thomas