There are CDMA networks in Pakistan that support Rev. B. The problem is the overall spectral efficiency doesn't go up all that much. Rev. B is kind of like Dual Carrier HSPA+ - taking two 5 MHz carriers (in the case of HSPA+) and using them as a single channel. The overall bandwidth for the sector is shown to go up somewhat and on an unloaded network a single user would notice higher (or much higher) speeds but the bandwidth is only going up somewhat.
Imagine this situation: an EVDO-A provider has a block of spectrum from 2000 MHz to 2005 MHz (for the downlink). Each individual tower has carriers at 2000.0 MHz, 2001.25 MHz, 2002.50 MHz, and 2003.75 MHz. There happen to be four users on that tower, user 1 is assigned the 2000.0 MHz carrier (which is the spectrum from 2000.0 to 2001.25 MHz, and ignoring guard bands) user 2 the 2001.25 MHz carrier and so on. Those four users would each see (given a good radio environment) theoretical speeds of 3.1 Mbps (minus overhead).
If there were only one user on the tower, he would still see a maximum of 3.1 Mbps - he can only use the one carrier.
If there were 40 users on the tower, they'd each see (presuming they're all using it at the same time the same way and have equal radio environments) 310 kbps and if there are 400 users on the tower (this is statistically pretty close to reality) it'd be 31 kbps per user if they're all using it at the same time.
Ok, so that's our situation: 400 users per tower, 5 MHz spectrum that's being used by 4x 1.25 MHz carriers when fully loaded would see 31 kbps per user.
If that service provider decided to upgrade to EVDO-B the situation changes a little. Those carriers (2000, 2001.25, 2002.5, 2003.75 Mhz) get bonded and users that sign into the tower are assigned all to the single bonded channel (they're still separate carriers but the single user data streams are multiplexed across all of the carriers). So now rather than the one tower (or rather, one sector) having 4x 3.1mbps channels it's got 1x 12.4 mbps channel. For those same 400 users (again, if they're all active) there's no improvement - 12.4 mbps / 400 users = 0.031 mbps / user, just like EVDO-A above.
That's not completely the case, each carrier on EVDO-B has a somewhat higher data rate of 4.9 Mbps in optimal conditions so it's really more like 19.6 Mbps/400 users = 0.049 mbps/user and the overhead per bit of the wider channel is lower so there is another improvement there.
The gain on an unloaded tower would be great - a single user would see 19.6 Mbps maximum instead of 3.1 Mbps maximum, the problem is that the situation you're trying to resolve by going to EVDO-B is a overall sector capacity problem, not a single user capacity problem, and it's not all that much improvement.
That's not saying there would be no improvement, but it wouldn't be the 3.1 to 19.6 factor that some people might be expecting.
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