In general it's all about "ain't broken - don't fix it" philosophy. What benefit will the upgrade give, except of bringing potential possibility to break any .pnd that uses python and possibly relies on older version somehow?
And what's the reason to update to 2.7.3 specifically, when latest 2.x series release is 2.7.6?
The benefit is clear :
Python 2.7 is compatible with 2.6 (I'm not asking for Python 3), so nothing is expected to break there, and on top of that all Python 2 programs nowadays rely on 2.7 features, missing from 2.6.
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Note that many Python modules rely and expect 2.7 too, and do not support 2.6 anymore.[...]