Instagram is killing twitter when it comes to short-form public social profiles. Depending on the hash tags, you will get likes instantly for your photos. The narcissism and instant gratification can be very addicting.
With twitter, you either need a re-tweet, or a "favorite," which clearly doesn't work well because no one wants to "bookmark" tweets.
To me this looks like an attempt to grab some of the "slacktivism" clicking that Instagram and Facebook provide by copying the metaphor with the lower barrier to entry.
To the newest kids on the block (in the literal sense), Twitter could position itself as "Instagram for Text." If that makes any sense, and it catches traction, you could see instagram responding by allowing posts without pictures, etc.
The closer these networks get to each other with form and function, the fiercer the fight could be. Which in Twitter's case is a good thing because they've been losing.