To be honest, while I'm on your side, every contract of employment I've ever had has a clause in it something like this. I don't think its an issue. It usually just means that if the company starts doing the same job in a slightly different way, or using new tools, or they branch out slightly, then you'll be expected to do what is generally in your job description, but maybe slightly different to what you're used to. It doesn't mean they can ask you to clean the toilets (unless you're a cleaner) or help them rearrange the office and move heavy cabinets etc. Health and Safety laws, as bureacratic as they are these days, mean they can't ask you to do certain things, even if you are perfectly capable, unless you've been trained to do it, and are insured to do it. Most companies won't routinely train and insure every member of staff to do every conceivable job, just because it is very expensive for them. To give you an example of what this clause typically means, I'm a computer programmer. That means I'm a desk jockey. However if the boss realises I'm capable of speaking professionally, then he may ask me to liase directly with a client to try to close a deal. Or a hypothetical example, if a window cleaner's boss discovers that the window cleaner is also good at cleaning people's front doors, and somebody asks for that service, then the boss can ask the window cleaner to do that. The boss can not then ask the window cleaner to go up on his ladder and mend the leaky roof, because he isn't qualified to fix rooves. This clause also facilitates career progression, because it enables the company to ease you into other duties without risk of breaching the contract.
With you all the way on what you said, Clueless! - bar the last sentence because it's too optimistic to think that Nat's firm has got any interest in furthering her "career" within the company. Like I said before it isn't a company that you or I would consider working for!
[size=large]I find the issue re: annual leave interesting. In hospital work, all staff got the same annual leave entitlement which for my grade was 4 weeks plus 9 bank holidays. If a full timer took a week off, it was 'worth' 37½ hours and if they took a single day, it was 'worth' 7½ hrs. It became complicated because they never worked 7½ hr days so a lot of fine tuning had to be done! For instance, I worked a 20hr week over 3 days, so I had to work out my annual leave in hours rather than days so if I took a week off it was 'worth' 20 hours. But I worked two 6hr and one 8 hour day so if I took off a single day, it would be 'worth' 6½ hrs, one third of my weekly hours. To compensate, I'd have to take the odd half hour on my other 6 hr day by leaving ½ hr early. Complicated, huh? But overall, I still got exactly the same number of weeks leave and public holidays as my full time colleagues. [/size]