We use a mix of Marketo and non-Marketo landing pages. For new inbound leads we use non-Marketo landing pages. For example, our Adwords landing pages are Marketo fields on a non-Marketo page. This gives us more flexibility with design and tracking as you mentioned in your other post.
However, we use Marketo pages (as iframes) on lead nurturing content. If we're sending an email to our existing database, we want to leverage progressive profiling to make form completion easier for the lead and to fill in our data gaps.
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