There are 2 schools of thought @lucy00
1) If it ain’t broke, why fix it? So some people prefer to leave a working site alone and not introduce risk of needing to change things to accommodate new features in the Theme.
2) WordPress and Czr undergo continued Research & Development to make them better/easier to use. Also improve any security gaps that may be found (cf Windows Update in PC land).
Myself, I prefer 2) and it is easier as I only support a handful of sites. Czr has recently passed through a major update 3.1 > 3.2 which will make it much easier for any new Users that come on board (> 0.75M downloads to date).
Nic is well aware of the need to keep backward-compatibility high on his agenda, but sometimes changes are necessary for the long-term support of the Theme.
The important thing to consider is how you can test these changes before you apply to a live site. I use Desktop Server which has proven to be the best tool I ever invested in.