In all honesty, from a front-end UX perspective - I think it's a marketing thing. Here's what I mean.
gmail.com redirects to mail.google.com - but, Gmail is easier to market and remember for a user.
Both googlemaps.com and maps.google.com redirect to www.google.com/maps
googleplus.com redirects to plus.google.com
From a server perspective, subdomains are a good way to isolate various configurations easily (and can actually aid in the user experience):
Apple markets the "Developer" program, which is connected to "iTunes Connect". And developers can create marketing for the iAd network using the "iAd Workdbench". And, last but not least on the list, if a user types in apple.com/developer - (s)he will be redirected to the subdomain.
And, there is always: ux.stackexchange.com, pm.stackexchange.com, etc. - basically every exchange in the stack exchange network except stack overflow, which has its own domain.
There is also the argument that it might be easier for a user to hit the period (ring finger and used more often - muscle-memory) versus the forward-slash (pinky and used less often). Also, visually, subdomains are better from a designer perspective than the slash.
And, there is the argument against having www. as part of the root domain.
To sum up. The end domain name is pretty irrelevant from a user's perspective, in my experience. Instead, the entry domain is the part you want to care about. gmail.com is a great entry domain - short, sweet, and marketable - but that's now where you go as a user. stackexchange.com as an entry will be a good one when the marketing picks up - then using the exchange listing - then bookmarking that specific exchange (at which point the domain becomes URL structure becomes moot). Domains aren't necessarily created to house the website - they may just be to help a user get to the right place (iPhone.com, iPod.com, for example).