I wonder if any one could tell me ifand whyI should upgrade from 10.0.25 to 10.1.012?
Well you might want to wait a few days, 10.1.013 is due out shortly.
But getting to your question, why upgrade from 10.0. If your 10.0 is working fine then it's probably not urgent.
If you have the Professional version, MKL had a major upgrade/improvement in the 10.1 Pro product. If you don't have the Professional kit then you don't have MKL bundled with the compiler.
If you want to use Ubuntu 7.10 you'll probably want to update
For support, 10.1 and 9.1 are the currently supported products. 10.1 superceded 10.0. So if you report a bug against 10.0 you may be asked to upgrade to confirm that the bug is in 10.1. There will be no more 10.0 releases and hence all bug fixes will go into 10.1 and/or 9.1.
And as usual, you will get some bug fixes and minor improvements with 10.1.013. See the Release_Notes for 10.1.012 or (later this week) 10.1.013.