Some points not mentioned (I think) yet...
The number 1 way to get links to a website is traffic...the more people that see the site, the more chance of it getting links, social shares etc. If PPC traffic & organic traffic (& other types of traffic) are not going to the same domain they're likely to get less links per domain, which reduces their chance or ranking well organically, which is contradictory to what you are wanting to achieve. If I were the client and I were getting links to non indexable pages, I wouldn't be happy. I would investigate if those PPC landing pages actually do have links/shares and if so use that to support your case i.e. those links could have been pointed at the domain you are trying to improve organic rankings for.
Remarketing via PPC is a huge opportunity via display and especially RLSA. If I was running their PPC it would really annoy me that the audience that I could remarket to is reduced because the traffic is split over domains and not consolidated in 1.
Analytics is really messy. I'm a firm believer that any web reliant business needs to understand and use the data available to them in order to grow, especially if traffic/online revenue is significant. Splitting analytics (GA or otherwise) and KPI's over multiple sites, analytics accounts etc is not conducive to managing and benefiting from that data.
Sub domains - might not be relevant, based on why they want to use off domain pages but could you suggest they use sub domains instead of different domains? This way they can still promote/operate separate sites but you'd have the advantages of 1) capturing link equity 2) increasing your remarketing audience 3) having all your analytics in 1 place.
