I agree. You should never mask a domain or share hosting for multiple domains on one domain. From an SEO standpoint it's suicide for you domain.
What you should do if you develop a lot of domains is to get a true multi-domain host or a reseller hosing account (which will be true multi-domain). Thre are many places that offer reseller accounts. Not too many places with true multi-domain hosting though. I know of 2 that I have used and liked, though, but the problem is they do not use CPanel but rather WHM. I prefer CPanel. But it seems that all true multi-domain/NON-reseller hosting accounts that are worth a damn use WHM rather than CPanel. Not sure why that is.
These 2 true multi-domain/non-reseller account hosts are...
http://www.neureal.com
and
http://www.existhosting.com
I like Neureal better, but they are both good. I no longer use either as I have a CPanel reseller account with another host now.
If I ever do require multi-domain hosting and don't necessarily need CPanel, I would go back to neureal in a heartbeat.
Beware of CPanel hosting that is not reseller but claims it is multi-domain. It really isn't. It is what you can add-on domains. Meaning, you can park unlimited domains and then have unlimited subdomains on your main domain then redirect/mask your parked domains to their respective subdomain on the main domain. Pointless and not good for SEO.