1. They want to corner the advertiser & business market - ie it's an attack on Hootsuite etc
2. They want to lock out competitors from pwning them on search (Google/Bing incorporating [good] twitter results into their searches would IMO be devastating to twitter - particularly if they didn't embed intents / hueg links to twitter everywhere)
3. They want to own the ecosystem so that twitter clients don't cross-post to Facebook/G+/app.net/favourite social network here
4. (I don't think this is likely) - Twitter thinking that they can somehow squeeze an extra couple of bucks out of each user if they're on an official client via advertising or something similar.
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Each of these seems plausible to me, but all of them essentially involve twitter holding customers/data/users hostage which doesn't seem like a great strategy.
Is there some angle I'm missing here / reading too much into?