sometime, there is a piece of wikicode i get from elsewhere and want to add to the page.
more often than not, this comes in the form of pre-filled template: e.g., in hewiki we developed tools (in the form of bookmarklets) that generate external-link templates from popular sites (the user activate the bookmarklet on some on-line news site, and the tool generates the appropriate template for this site, with article name, author name, date etc. already filled).
another example is the "coord" template: we created a bookmarklet that allows the users to generate pre-filled "coord" template from google maps.
there are other cases where the user might want to inject some prepared wikicode into the page when using VE.
create a button/hotkey that will open a "enter wikicode" form. it's ok to require that it will be legal wikicode and ask the parser to test it, and it's ok not to do so. the important thing is the request to allow insertion of raw wikicode through VE, rather than requiring that i'll save and switch to source code editor.