There are at least two reasons why you might want to work somewhere with a high percentage of people who've been there for a decade or more. In the first place, a high percentage of long-servers suggests your job might be more safe - you may not have a job for life, but you could have a job for 10 years. In the second place, the greater predominance of old-hands suggests more in the way of culture and 'institutional memory' - those nebulous things which are supposed to prevent cultural dissolution and new hires from running amok.
It's interesting, therefore, that there are big discrepancies between the proportions of long-serving staff working at different banks in the City of London. Analysis of the start dates of FSA Approved Persons (ie. persons approved by the regulator to perform certain controlled functions) shows that some banks (Rothschild, UBS Ltd, Credit Suisse Securities, Deutsche) have a high percentage of people who've worked there for 10 years. Others (Barclays Capital, Merrill Lynch International), don't.
Does this matter? Barclays has hired copiously in the past decade, adding 8,000 people globally between 2007 and 2010 alone. Its low percentage of long-serving staff is a reflection of this. Equally, the past is not necessarily a reflection of the future: just because you join a UBS, a Credit Suisse or a Deutsche now, this doesn't mean you're likely to still be there in 2023.
However, Barclays' investment bank may well be on the cusp of some big changes. Institutional memory and a strong culture could help pull things together. Bob Diamond and Jerry del Missier were Barclays' culture-carrying old timers, each having worked at the investment bank for 16 and 15 years respectively. But Bob and Jerry have gone and now there's just Rich Ricci (18 years' service) and less than 100 other FSA registered long-servers in London, along with several thousand ex-Lehman bankers in New York. This doesn't sound like a recipe for cohesion. But it may be an opportunity to build something entirely new.