Like Keith says make sure you mention it.
The ECO will be looking for skills like this as most Wifes will want/need to work once here in the UK.
If it helps we put my Wife had passed an English qualification on our application.
The other reason i can think of is it shows that your wife can speak English to a decent level. Sounds silly but if she couldn't it might be thought unless you have another common lanugage, that there is something dodgy going on lets bring her in for an interview.
