Yes, who cares if you get extra testnet coins?
But... if somebody wanted to be annoying, they'd pre-generate as long a min-difficulty coinbase-only-transaction chain as the block timestamp rules allowed, and constantly broadcast those blocks. Just to prevent transactions from getting confirmed.
To prevent that...
Testnet could prefer to build on blocks with more transactions from the memory pool over blocks with fewer transactions from the memory pool (that's not a bad rule for main net, either; might be worth considering if it works well for testnet). The rule now is "build on first valid block seen".
And "discourage" (refuse to directly build on or relay) blocks with timestamps in the future.