Sydney, not only do you not need a plugin, a plugin can actively hold you back. I encourage clients to move away from CMS-hosted forms (Drupal, WordPress, et al.) because such plugins are necessarily rate-limited or, perhaps worse, create a DoS attack vector against the underlying Marketo instance.
One client on Drupal broke their other Marketo integrations after a highly successful trade show, since their Drupal plug-in -- otherwise well-thought-out, but in this respect disastrous -- used the rate-limited REST API for every post. (Their older plug-in used the Marketo forms endpoint, and that would've been just as bad in a different direction, since they would have missed more form submissions!) Most people are one lazy teenage hacker away from a broken site, without knowing it. Working in part in the enterprise, where those threats are very real, I can't live with situations like that.
Some look at this as a cosmetic decision, since there are very pretty form themes available for CMSes. But that's a different question. You can use the Marketo Forms API (from the browser) to submit data from non-Marketo forms. You thus get the best of both worlds: the aesthetics you want without the unprofessional rate limits. No CMS plugin necessary.
Another point people make is about form PreFill. But you actually don't need a plugin for that, either, and the explicit reliance on real-time Marketo API access shared by all (as far as I know) plugins is a poor design decision.
A theoretical plugin that might be attractive, on the other hand, is one that uses the Marketo Asset API to read form definitions (not to post forms, only used when you're in the CMS control panel) and is able to theme up Marketo <FORM> tags natively. It would still use the Marketo embed code and post directly to Marketo from the client. I've never seen anything like this in the real world but it would be cool
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