Few reasons...
- We didn't like new players first joining internet training servers (a natural first step), to see the vBF2 style spam that happened on them, giving them a false first impression.
- We didn't like people playing training mode just so they could do vBF2 style spam, and mess up honest people actually trying to train.
- Co-Operative mode offers the training needed to be able to "safely" enter an online server (as well as reading the manual). You can practice flying planes and helicopters without the worry of wasting assets, and you get someone to shoot at in the process that won't hate you for killing them over and over.
As for the problem of SP vehicles handling differently than MP vehicles, that has been fixed, you can only enter MP air vehicles as pilot (you can still enter SP vehicles, just not fly them, since they have vastly different handling, giving a false impression)
- One complaint is that what happens if you crash/die in your vehicle, it takes longer to respawn in coop. Well if on a local server, you can simply type "admin.restartmap" into the console, and you get all you're wasted vehicles back straight away

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If on an online server, obviously you can't do this (without admin support anyway), but you should train first how to use the vehicle without dying (it's mainly air vehicles that's the problem here) in a local server, then for more advanced tactics with multiple people (such as gunning, tanking, etc), then go online, and this shouldn't be a major problem. You can always go infantry while waiting for your vehicle to respawn...
TL;DR
We feel that Co-Operative gives a better training environment than the old Training mode for new players. Training mode will never be coming back into PR. If you really want training mode back, there are community made maps that offer a similar experience.
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Oh, and for those who honestly didn't know, a "local server" is you creating a game where only you play. If you're set up on a LAN or through Hamachi, others can join you, but lets not go into that right now. Basically, it's BF2's way of "singleplayer". You do not need a PR server license to make a local server, you do not need server files to make a local server. Simply choose it in the menu, then select your map and go create game, and you're away.