It's always benefitial to know the previous versions, but that would be more helpful if you are in support role or migartion project and you are willing to engage in roles where you need to work on 2005.. If you don't anticipate these cases, I'll suggest not to go for 2005. You should have exposure to 2008 because you may have to work on it as not everyone is using upgraded version. To summarize, get a feel of 2005, majorly limitations. No need to dive deep into it. That's what I think 😉