Actually benefiting from the motion requires a new algorithm. Probably not that hard to design, but not trivial either.

As always CAPTCHA is a game of cat and mouse. When something is completely broken, you need to think up something new.

With this implementation you don't, but it's not hard to construct it such that there is no single frame with enough information to construct the whole message. If you screen capture there are several frames where you really can't tell what the middle letter is, but it's obvious what the message is.

The real issue is a computer can compare the images individually and combine them so if it get's 'G_G', 'GF_', and '_FG' it's going to assume 'GFG'.

In that case you don't gain anything from the motion (except maybe the ability to vote between your bot's output for the individual frames, but that also makes your bot 20 times slower).