I am now using a mix of Mac OS versions 10.6 and 10.7, and the main motivations I have seen for upgrading beyond OS 10.6.8 are:
1. Security: since about mid-2011, only OS versions 10.7 and above have been getting full security update support, including a recent security update of Safari. However, OS 10.6 did get a security fix for the much discussed FlashBack trojan a few months ago, and other browsers are still getting updates for OS 10.6.
2. iCloud integration seems lacking compared to the newer Mac OS versions.
Decide for yourself whether any of those is enough reason to spend $19.99 on the upgrade, and grapple with any software compatability issues.
I have a question though: if one needs to go beyond 10.6, for reasons like an employer policy of only supporting versions that get all security updates, is it helpful in some cases to go only to 10.7, in cases where 10.8 does not yet fully support your software? This is still possible with a phone order.