Re: Any orbit should do
There is a pretty much set overall mass for your payload, given that your launch rocket has a payload limit.
Every extra kilo of fuel you carry is one less sensor package on your probe. Or less shielding / backup battery capacity / whatever.
If you want to decelerate the whole package to get into some sort of Martian orbit - then you've got to carry quite a few kilos of extra fuel. That means you do less science because either your probe doesn't last as long, or your probe has fewer instruments.
Worse, it costs even more fuel to get out of orbit - because you have to reduce your velocity quite a lot.
Basically it's a waste. And with our limited technology for getting out of the gravity well, we just can't afford waste in space missions. At the moment SpaceX charge $60m-odd to launch a rocket that's only burning about $300,000 worth of fuel. And that's because they're still pricing on throwing away the rest of the rocket.
Remember, it still costs $10k-$20k to get 1kg to low earth orbit - and in that total mass you still have to budget for all the lovely fuel to get you to Earth escape velocity, do a few course corrections and slow you down enough at Mars to make aerobraking workable in a very thin atmosphere.
On the other hand, the mass of a cubesat is very low compared to the whole payload. So even carrying the cubesat's entire mass again in extra fuel probably isn't an insurmountable cost - and whereas that fuel doesn't get you very far boosting the whole payload, it might take your cubesat quite a long way. And of course it's required (not a luxury) as you have to put your cubesat in orbit - putting your probe in orbit is an expensive luxury. Much cheaper to keep it in storage, in a nice clean-room on Earth, and send it after the dust has settled.