1. to have the planets of choice. By having more jump, you may choose them better
1. Doesn't matter that much. Lower jump numbers just take more time to get there.
I disagree.
The simple example of the Collace/District 268/Spinward Marches Jump-3 opportunities are exceptionally compelling. You basically have an Industrial world with 2 Agriculture+Rich+Non-industrial worlds and 2 more Rich worlds within 3 parsecs of Collace and only 1 of those worlds is adjacent to Collace at a distance of 1 parsec. By literally
not wasting time and overhead costs on the systems in between these extremely compatible (in a lucrative sense) markets, a Merchant Prince ought to be able to maximize their profits most handsomely, particularly through the REAL moneymaker ... speculative cargo. The Jump-3 drive on the Subsidized Merchant then enables a "maximum revenue density per unit time" opportunity through every jump, rather than getting stuck with a "just passing through" jump (or 2) before being able to buy/sell fill up your manifest again with passengers, cargo and speculative cargo again.
2. being large corporations, they are not subjected to mortgage, aas they have money enough to pay it
2. They're still concerned with costs. Opportunity cost of capital figures in here.
Not needing to pay the usury rates of bank financing (pay for 2.4 ships in order to own 1 ship) does amazing things for the profit margins of a wide variety of ship classes. Some that are marginal at best on bank financing can be wildly profitable either under subsidy or when paid off and merely needing to account for operating expenses (life support, crew salaries, berthing fees, fuel, maintenance, etc.).
according MT:SOM, those large companies take more money from speculation than from cargo/passenger service
3. So the spec cargo goes by fast movers; the bulk cargo goes by the cheapest method.
Not to put too fine a point on things, but Adventure Class Ship hulls fall into the "fighting over table scraps" category after the "big boys" have had their fill gorging themselves on the economies of scale. And as pointed out, speculation can potentially be WILDLY profitable (just not on a predictable schedule or to a predictable degree). As a Merchant Prince it is your fiduciary duty to seek out the most lucrative opportunities to turn a profit (and expand your line of business), which means that speculative cargo is all about "stacking the deck" in your favor depending on the route you're planning to run. Set yourself up to "jackpot" more often Over Here than somewhere Over There and you're on your way to growing your wealth.
they probably have some kind of subside by the Imperial authorities, as they are the "main railroads"
Depends on the strings being attached with that subsidy. Mandatory conscription/mobilization in the event of a war being declared can sometimes seem like too high of a price to pay for people who prefer to choose their destiny, rather than having it forcefully thrust upon them.
To be honest, the megacorps have gotten SO LARGE in the OTU that they in effect don't "need" subsidies anymore, but if they're on offer they'll go ahead and take them ... partly to keep a competitor from getting it (and getting established as a rival concern).