My question is straightforward: Is it considered a bad idea to use a ghost button for an action such as login? As far as I have read about it, the ghost button is meant for secondary actions. The fact that it's less visible compared to a solid button, it won't grab the attention of the user as much as a solid one would and might be a bad choice when you're looking for a "call to action" such as logging-in. Anyway, I would like to hear from you guys.
It sounds like you have already answered your own question. Is there a specific reason you're wanting to do this?
With no further information, I'll just restate your question in the form of an answer.
Ghost buttons are meant for secondary actions. Logging in would likely be a primary action, so this should use a primary button style. You as a designer know that most users, after filling out their login information, will be clicking the "Log In" button next, so why run the risk of making them search for it?
Additionally, web-based buttons have traditionally had a 3D aspect to their design, either with shading that makes the button look like it protrudes from the page, or a drop shadow that shows it "hovering" off of the page. Ghost buttons, being flat and having only a border, look less like traditional web buttons, so less web-savvy users (i.e. most of them) might not recognize that it is an interactive element.
In short, do the best you can to minimize your users' processing time. If your page is quickly and easily understandable, it is likely to provide users with a better experience.