1. its a bit easier to not include txs
Negligibly so. It takes a small fraction of a second for miners to build the merkle root when there are transactions in the block.
25 BTC is issued per block to encourage miners to support the network verifying transactions. But what the sense of awarding miner in this case?
Every new block added to the blockchain makes it more difficult to modify all the previous transactions that have ever been added to the blockchain in the past.
The main purpose of the block subsidy is to provide a reasonably fair way to issue the nearly 21 million new bitcoins. It is convenient that this subsidy also acts as an incentive to mine when there aren't very many transactions yet. Approximately every 4 years the block subsidy will be cut in half. Eventually, the value of the transaction fees will be much larger than the block reward itself, so the incentive to mine empty blocks will get smaller and eventually vanish.