Why is pruning not considered already at the moment? I understand that Bitcoin scales in several meanings (Scalability), and pruning is one important concept of it (SE Question). I also understand that a "Simplified Payment Verification" (SPV) needs to trust a lot the peer he is obtaining the block chain from.

A very conservative pruning (e.g. transactions older than six month and spent) wouldn't do much harm, especially when it would be only a configuration option for bitcoin-qt. That way the default is the full node, but it's easy to have a "small node".

But I don't see it coming anyhow soon. Is there a reason? Is it so important to have full nodes at the moment that the devs say "either you go for all or nothing"? Or is the development effort the bottleneck? IMHO a large network of non-SPV nodes is more important than a small network of full nodes.

Edit: Let's put it more concrete: Is there a major security issue of not having the complete transaction history of the world back to the genesis block?
Pruning is being considered, in fact, it was taken into account when designing the 0.8 database format. The unspent transaction outputs (which is the only essential piece of data necessary for validation) are already kept in a separate database, so technically removing old blocks is perfectly possible. It'll likely require some small changes to make sure the code doesn't break when block data that doesn't exist anymore is requested, but this is easy.

The reason it's not implemented is because of the effect on the network as a whole. If a large amount of nodes starts pruning old block data away, it will become harder for new nodes starting up to find the historic data to verify. This is not a problem as such - I expect enough copies will remain that this is not a true problem - but we need a discovery mechanism so nodes don't need to arbitrarily try peers until they happen to reach one that has the blocks they need. In fact, there's a discussion going on on the bitcoin-development mailing list about this right.

EDIT: Pruning was implemented in Bitcoin Core 0.11, and is fully functional since 0.12.