I think there is a plugin for C2, that makes sprite sheets (with uneven width/height) possible...? Can't remember where I saw it though, and I have no need for it as of yet...
Sprite sheets are efficient in that they require far less file calls, than if they were loaded in separately.
Imagine that you have 100 sprites, which is quite a small number anything but the simplest casual games; now imagine 100 file calls to either buffered RAM, or even worse; the hard drive. If these 100 sprites were consolidated into ONE file (the sprite sheet), you'd gain a lot in performance from lesser file calls in and on itself.
But any file has a header; and that header takes up space. Imagine that the header for each file, is an average of 5 kilobytes. Now multiply it with 100, and that's the extra overhead you get from not using a sprite sheet.
And it gets worse: If you're making an online game, then your server delivers the content; which means your server will have to fetch the 100 files. As most servers features traditional hard drives (magnetic), the search time is very high (compared to SSD's) and your 100 files will have a noticably increased load time, compared to if they were 1 sprite sheet.
But here's the thing about Construct 2:
All sprite animations are automatically consolidated into a sprite sheet - no matter if you pulled them into Construct 2 as separate files or as a sprite strip. Go take a look at the exported files; you'll see what I mean. And that's pretty clever.
My first game in Construct 2, has 35 graphics files, but these contains several automated stripe sheets, and they actually consolidate a little more than 50 extra graphics files - so without this smart feature of C2, my game would have contained a little less than 100 sprites.