As nice as Virtualdub is, it's quite old now and cant read newer video files, like mp4 flv mkv etc..
I like it because it does a simple job very well with no unnecessary bells and whistles.
It adoesn't complain when I want to output bizarre uncompressed video formats like 800x1 pixel monochrome, 16 bit 5:6:5 colour or (as I'm doing right now) 4x1 pixels RGB with a mono 48K audio.
I can also hack together a filter for things like error diffusion dithering for smoother 5:6:5 displays by modding an example filter without needing to know too much about the plugin architecture.
I can just tell a customer to go download it (no installation needed) , set a few settings & it just works.
Maybe there are other options out there but if it ain't broke....