There are plenty of reasons for it. One or both were PC'd and can't be combined until one is paid off. One is bucketed, but is one of their oldest cards so they don't want to close or combine it. They applied to get SUBs.
Same could apply for other cards. Why would you want 2 discovers? First year cash back match making it 2-10% cb. Combine cards at end of year, wait until you can apply again, rinse and repeat.
For a while I had three Capital One Quicksilvers, for the reasons others have already alluded to.
My Platinum MC is my oldest bankcard (by years) and I don't plan to get rid of it ever. It started off as an Orchard Bank/HSBC Platinum MC with 2% rewards but a $79 AF (!). When it changed to Capital One they made it a QS1 with 1.5% rewards, but kept the fee the same. I eventually got an upgrade to a QS which took away the AF, then a couple of years later as an experiment I changed it to a Venture One to see if I would be upgraded to WMC (I wasn't). Finally I PC'ed to a Savor back in July, literally weeks before the refresh, so the account is now a grandfathered Savor w/ no AF. (I'm still surprised how lucky I was there...)
Then I have a Quicksilver Signature Visa; it has the highest credit line of my three Capital One cards, but it never gets BT offers. I opened it initially to take the place of the Platinum MC since I was afraid the MC would never be upgraded (but then it was). I have an AU who now uses the QS Visa Signature as a 'daily driver', and I specifically chose it since Capital One has never bothered me with fraud declines - it just works - which in this case is more important to me than slightly higher rewards.
Finally, I have a QS WEMC; it started off as a Venture (I wanted the bonus), and I PC'ed it to a Quicksilver before the AF came due. It's been getting nice BT offers that the other two cards don't get so it's been useful in that way. Just a couple of months ago I took advantage of a 2% fee/18-month BT it was offering, which beats my 'standing offer' with BoA (which is a 3% fee).
Since all three cards are useful on their own I'll likely keep all three for the foreseeable future. If I were to combine/cut one lose at some point in the future it would be the QS Visa, which ironically has the highest CL as well ($17k). That would create another dilemma, since I don't need a Savor with a $20k+ CL, and if I were to combine it with my WEMC the limit would likely put me on the radar for a CLD at some point, so I'll probably just leave them all alone.
TL;DR... there are legit reasons to have multiple Capital One cards (even the same rewards type). 
No reason not to.
No AF or FF.
Can compine later if wanted.
Can PC later if wanted.
Cash back.
When you PC the only thing you're changing is the rewards structure. If you PC from a Platinum to a savor, it's still a Platinum and will grow accordingly. The visa and mc aspect is moot as it will depend on when you PC. The only time you'll have an issue is when you PC to and from an AF card. As for a savor upgrade, it's generally not offered. You have to call in to ask for it when you have an upgrade available.
