Magento finally release version 1.9.1 if you compare latest magento and older version of
mangeto you can find lots of updates on it. It can be security , performance , Functionality
and UI updates. Here I am elaborate reasons to upgrade magento stores.
1 ) Security : As magento is well know open source ecommerce
hacker are eager to exploit magento security.If you go through thehackernews.com
you can findsecurity issue in magento below are some of the link
http://thehackernews.com/2015/04/Magento-security.html
http://thehackernews.com/2014/02/Magento-vulnerability-Administrative-User_13.html
Magento team is work hard to keep mangento secure and updated code day by day.
Now think if you are running store on magento out dated version hacker can easily
hack your store.So kee you magento store updated to avoid unethical activity on
your store.
2 )Features enhancements: Each new version of mangento came with some new
feature which will help you to run magento store efficiently and better conversion rate.
Magento 1.7 new feature list :
EU Cookie Restriction
Auto-generation of Coupon Codes
Filtered Navigation Pricing Enhancement
Customer Group Pricing
CAPTCHA
Payment methods (COD)
Magento 1.8 new feature list :
Enhanced Tax Calculations 350 functional improvements that boost product quality in key areas,
including the web store,shopping cart, admin order creation,
import and export functionality, web API components, and payment methods.
Magento 1.9 new feature list :
Responsive Web Design
Payment Methods improvement
Click here to know about new feature in magento 1.9 .So keep your store updated and take
advantage of new features.
3) Magento performance : Each new version came with performance improvement.
Check magento version 1.8 release note . If you are running with older version of magento
so you can not take advantage of performance improvements done in newer version.
4 ) Error Fixes : There are some known issues are in magento. Click here to know current
issues in magento. New version came with less issues so keep you store updated.
5 ) PHP new version support : PHP 5.4, which provides better performance and more
efficient memory usage. For more information, see the PHP changelog.
Magento Developer Blog 