Sorry but i think is not obvius!
1) -You spent more components
2) -You spent one extra port pin
3) -You need extra code for activate - deactivate the circuit
4) -What hapend if you forget deactivate the circuit?
A nice burned circuit?
5)If not then means that your citcuit is bigger.

1) Yes, but you save a heatsink. The extra components take less space than a heatsink and - important - you will not have a heatsink to touch, possibly at 300VAC. Also, you can use a smaller device for the cc generator. I would, at any time, rather avoid heat than radiate it away.
2) so what, with internal flash and RAM, there is plenty of them.
3) abt 10 lines.
4) yes, but do not 'forget'. Anyhow you can test the circuit with 25V in then it will not burn if error.
5) see 1)

Erik