It got us lots of users (0.25B monthly actives) but they are (relatively speaking) quite poor, so ad revenues aren't as high as you might wish. So you need a lean operation, at least if your operating cost is significant - like transcoding web pages for 1/30th of the global population...
But its a often a low end phone with slow internet speeds.
If I remember correctly, one of Google's advantage over their competition in their early days was their page would load quickly on low end desktops and slow dial up connections.
Not only should app developers use slower internet speeds, they should be developing it for the low end smart phones- Which make up for most of people who are using smart phones.
Especially in a country like India, there are a lot of people using $20-$30 smartphones. Who make most of the users out there. Develop for them, and you get a huge user base for free.
Edit: It seems to me that depending on the market one targets there's not much to be gained from designing for slow connection speeds...
For example, on my trip a few weeks ago, I used T-Mobile's free 2G roaming to get around. Searching for something on Google was pretty quick, and loading a Wikipedia page also was relatively fast. But the second I moved away from them and onto real sites that didn't try to optimize for lower speeds, I was pulling my hair out at how much data was trying to get sucked down on such a slow connection. It took minutes to load some websites.
Return visitors to your page/app/whatever?
Just because one is connected using Verizon, doesn't mean the connection speeds at any given time are all that great.
Otherwise, you might figure it out when you suddenly lose most of your users to a competitor who offers a snappier experience.