The need will become more apparent in the next couple of years as people who buy 64 bit machines will most likely want a 64 bit version of Windows on it. I know I will be when I upgrade to Vista in the next month or two so I will most likely have to get my older laptop out to run mIRC on.
The advantage? Well that is a different issue and perhaps there is none at this time. One thing is for sure - mIRC has a huge market share and that is unlikely to be dented in the short time by the lack of a 64 bit version. It would be nice to know what Khaled has planned in this department.
