X and Nexus still lack one feature which is expandable memory
While expandable memory is a good thing, lack of expandable memory is not at all just a bad thing. And there's a single most important reason for it. "Performance". Speed of internal memory is usually a lot higher than external memory. While a class 10 card provides a sequential speed of 22.62 MB/s (taken from a review on FK of Class 10 card), a NAND chip of modern device like Nexus 5 can provide Sequential speeds uptp 76 MB/s (Source: AnandTech | Google Nexus 5 Review).
[I know comparison isn't that scientific but I am just making a point.]
So when a high end game like Asphalt 8 needs to access files kept on storage which memory do you think would provide maximum performance?
So there are many factors which makes Internal memory a good thing:
1. Speed / Performance: Already discussed.
2. Security: External memory can't be protected by software. Sure you have App locker that seems to lock access to files for unauthorized users, but what happens if the SDCard of your phone falls in the hand of someone who wants your data? He will be able to access all data kept on the card easily. This is the reason why some apps like Banking apps just can't be moved. (A good thing).
3. Usability: If you have moved your apps on card, then what happens when you unmount the card? The app on phone will become unstable and force close, if you haven't stopped it from running beforehand. Sure you will take care to unmount the app before unmounting the card, but an average joe won't realize this, thereby blaming Android for providing an unstable OS.
4. Widgets: Any widget that requires the app to be mounted will crash if you remove the card. Hence Google just decided not to allow any widget apps to be moved to card in the first place.
For a good read I would recommend reading these articles:
Which is better: Internal memory or external?
*android.stackexchange.com/question...-any-other-advantages-or-disadvantages-to-sto
So with all these quirks and perks of internal and external memory, Google just decided to provide no expandable memory. As I said not at all a bad thing if you buy a device with sufficiently large memory of 16 GB and above, and when the device support OTG. While what you gain in the end, is Performance like that of Nexus 5.