Your resume is a marketing document and you should treat it as such. Text has the advantage that it's universally supported, but it has the downside that it lacks the formatting tools that allow you to design your CV to highlight key areas.
Remember that someone reading your CV isn't going to read every word of it, they're likely to scan quickly through it looking for key points. While in Doc/PDF format you can use typographical tools to aid this (font size, line width, layout, etc.) in plain text you have far fewer options.
This of course assumes that you can put together a well designed CV, obviously a plain-text CV is going to be better than a CV written in size 20 comic-sans (I've actually seen this).
Remember that someone reading your CV isn't going to read every word of it, they're likely to scan quickly through it looking for key points. While in Doc/PDF format you can use typographical tools to aid this (font size, line width, layout, etc.) in plain text you have far fewer options.
This of course assumes that you can put together a well designed CV, obviously a plain-text CV is going to be better than a CV written in size 20 comic-sans (I've actually seen this).