The reason is at times I like to Solo the Elite Quests coz
The heroic quests are designed specifically to encourage or require multiple people grouping up. Asking for an additional companion just because you want to work *around* that limitation is just asking for TOR to become a single player game. If you want to solo heroics, you've either got to become a better player (so that you're good enough to actually solo them) or a better person (so people will actually feel like joining you). Of course, sometimes, if you're playing at awkward times of day (re: late night, afternoon on weekdays, etc.), you pretty much never have anyone on that you can group up with, but , for those, it's pretty useful to have a guild (I'm pretty much always available to punch H4s for guildies if I'm not occupied elsewhere).
3.Sometimes the Bosses in your Main quest hit so Hard that it's Hard to kill them When you are at the same level as them
If you're having problems with the boss of your Main Quest, you're doing it wrong. Pretty much all of the bosses are designed to actually be difficult if all you're doing is running up and deciding to punch them until they fall over. Many of the bosses have specific abilities that, unless you interrupt them, will instantly kill you. A few even have items that are provided to you when you get the quest that make the fight ludicrously easy. If you're having problems with the Main Quest boss, rather than asking for more companions so you can run around like it's a single player game, try asking for tips and tricks from the relevant class forums. I can pretty much guarantee you that there is at least 1 thread in each of those detailing how to deal with each of the Act bosses. Plenty of people have completed the class stories without vastly overleveling or overgearing the quest or bringing in allies to finish it for them. The Bosses are designed to be bosses, not just another elite standing in your way on the path to 50. Try to figure out what you're doing wrong rather than just complaining that you can't do it.
Also, if the developers allow the presence of 2 companions while solo, you're also asking them to increase the difficulty of the solo game (re: leveling and dailies) to compensate, not to mention making what little world PvP occurs even more chaotic (you're pretty much increasing the number of effects and models by 50%). In all, it's not really all that likely to happen because the game was designed with a single companion in mind. I'm not even sure if it's possible to have an degree of control over companion AI when you have 2 out (you can have a guest and a companion, but you don't really control the guest at all; I am curious if/whether a companion turns into a guest whether they keep their preexisting stats and abilitiy configuration, however, since that could address the "problem"). Probably the only way it *would* get implemented would be allowing a second companion to join you during your Heroic Moment (likely a random available companion or last companion you summoned before your current one) or simply reducing the DPS/healing/survivability of the existing companions by roughly half to compensate for the ability to have twice as many up at all times.