A hard truth about using Marketo is that you won't always use Marketo for email. At some point, you'll use MailChimp or SendGrid for one-off sends, or rent your list (with full opt-in of course) to a partner, or use an in-house mail blaster app for special events.
(On the blaster note, shouts to Gammadyne Mailer, possibly the ugliest best-of-breed app ever created, which did the trick quite well in the pre-MA days!)
You can only dodge this question for so long: Is there any way to track clicks on these emails if they're not sent through Marketo? If the addresses are already in your Marketo instance, yes, there are are a few ways, with varying levels of complexity + client-side slickness (the harder you work on the back end, the more streamlined on the end user side).