We have assemblies thatare to 6,000 to 12,000 components large or more. Opening these size models is not really possible in master rep so a computer that has more memory is required if users want to work with master reps. Since there are limitations to 32 bit physical memory, that's where 64 bit comes in. With 64 bit you have more memory therefore you can open more, that's ultimately the benefit as well as the benefits mentioned by 'omarhernandez'. We have made it our standard (we phased it in) and have seen good results, but mainly in our added abilities, not so much in terms of speed as it is only a noticeable click faster for typically modelling/drafting operations (not incl analysis).
As far as your statement about it being slower, it certainly isn't slower but it isn't mind-blowing faster either, so if you (or comapny) can handle the budget for 64 bit (it can get pricey the more memory you add), then I would recommend it to get the added ability. Although if you don't work with large models or analysis or if your idea of large models is 1000 to 2000components then I would consider holding off (cost-wise) and make good use of simplified reps until the cost is more desireable.
Hope this helps.