I see this as basically one of two conditions:
1) Use any external CMS to build Landing Pages:
PRO: LPs might be a little easier to "build" using a different editor than Marketo's (which is at least a generation behind most "modern" CMS systems like Unbounce or LeadPages)
CONS: they'll require you to build and manage an integration at some point.
2) Use Marketo to build Landing Pages:
PROS: You won't need to create custom integrations or manage syncs between two systems.
CONS: The editor is a bit less user-friendly (depending of course on how flexible your template system is).
I'll mention that I'm of the mind that anything you could do in Unbounce (or anywhere on the web), you can do in Marketo as far as the end-result of any landing page (look, feel, functionality). If it were a vote/contest, I'd vote for Marketo LPs for me, but there's a different "right" answer for every situation.
The trade-off as I see it is that using an external CMS lowers the barrier to entry for creating and maintaining new layouts and changes to your Landing Pages (so they say, I'm looking at you Wordpress) -- at the cost of needing to integrate the two systems, which generally requires more dev work to build, maintain and troubleshoot if/when trouble arises. If you've got an expert in house and they need something more to do with their time, this tradeoff might not really matter that much -- and assuming everything works as expected.
Personally, I think less is more here, and if you can get away with fewer integration the better off you are in terms of data fidelity and being able to trust that what you're reporting on is "native data" rather than a skewed version b/c of some sync hiccup(s).
Another thing I might think about in this consideration is the rising importance of systems security -- the more parts you have, generally the more risk you'll have. Depending on how you plan to use Marketo (in-house vs. outsourced) it might be better or worse to use Marketo LPs. I'd think if you've got another team building pages on your behalf, it might be easier (less red tape) to have them work in an external system like Unbounce or LeadPages and just deal with the technical hurdles of integrating the two. If you've got folks in-house that will be using Marketo on the daily, I'd think it was easier (one system, not two logins) and more secure to go the Marketo LP route. Of course, there's exceptions to this based on your situation and security setup, but it might be something worth considering if you're laying a path for the long-term.