I've thought about this and have mixed opinions. Ideally you would like to see Bush's entire team stay with him for both terms in office. It's perhaps easy for us to look at it as why wouldn't you stay the course but we also don't live in that super pressure cooker environment. It is expensive to live in good areas inside or outside of DC. Those aren't normal 8-5 jobs and your family does suffer by you working extremely long hours, being on-call practically 24-7 and the frenetic pace at which much of the work is done. I can see why people would possibly want to leave after a few years for their families' sake and for health reasons.
And while it may appear to be fodder for conspiracy theories when key people leave before Bush's term expires, those same people can obviously make a much more lucrative transition back to the private sector while Bush is still in office. They cannot lobby during that time period but any company with a brain is going to realize they can give extremely useful advice to their company. The former Bush staffers can also make quite a bit of money on the public speaking circuit while Bush is still in office but demand for them speaking will subside as Bush leaves office and a new President takes office. The sooner they establish themselves as valued speakers, the better it is for them financially.
While they may make a relatively large salary (I believe $168,000 a year was mentioned), government FERS retirement is based on 1% of your average salary for the highest 3 consecutive years of employment tames years of service. A minimum of 5 years is required to become vested (i.e. eligible to collect retirement) and a minimum age of 55 to 62 is required, depending on the year in which you were born.
Using Tony Snow as an example, his short tenure (less than 5 years) would not make him eligible to collect FERS retirement even if he stayed until the end of Bush's term. Even if Snow had retired after 5 years (using his current pay scale of $168,000), his retirment pay under this FERS formula would be 1% of $168,000 ($1,680) times 5 years of service for a whopping total of $8400 a year. This emans he would still be under the poverty line and definitely need to get a retirement job. And in case you were wondering about how long it would take him to rack up a $100,00 a year in retirement, it would take him approximately 59 and 1/2 years of working that job.[:'(]
http://www.opm.gov/retire/html/faqs/faq11.asp
http://www.cpol.army.mil/library/benefits/docs/fersinfo.pdf