You're thinking in Earth terms, though. Generals, kings, presidents (whatever) are not on the front lines. But perhaps they do things differently on Nirn. I'm almost certain about reading some of Tamriel's books where they mention the leaders joining in. SubRosa? Help?
...And it's also possible that in the entire history of battles on Earth, occasionally there were leaders who joined in the battle for whatever reason. I'm not an expert historian. SubRosa? Where are you on this one?
If you go far enough back in history, kings and generals did in fact fight in the front ranks of their armies. Alexander is one example, Epaminondas of Thebes another. Of course the downside of this is that they gave up any hope of controlling the battle. They made a plan with their lieutenants beforehand, then went off to fight and did their best to make it work. The other downside of this is that when they get themselves killed, the war is over, even if their troops go on to win the battle without them. Epaminondas is a good example of this. Thebes went from being the power in Greece, to being nobody, becuase he got himself killed while defeating the Spartans. Alexander too, suffered numerous wounds from slugging it out alongside the rest of the cavalry. He was nearly killed in a siege in India, and his army actually thought he was dead, as he went down in front of their eyes. The toll of those wounds doubtlessly contributed to his early death. As the saying went, death was life's way of telling Alexander to slow down.
This all changed by the time of Hannibal (just a century after Alexander) whom we see not fighting personally, but leading and coordinating his army from the back of his elephant. Likewise with Caesar. Interesting thing about him is that when his army was ambushed at Sabis, he actually joined in the fighting twice, and each time his men pushed him back from the front line so he would not get killed. Still, even leaders as recent as George Washington were known to put themselves conspicuously near the front where their soldiers could see them. They were not fighting themselves, just getting a look at what was going on up close, and of course inspiring their men (it is good for morale after all). Washington had several horses killed from under him, two at Monmouth alone, and often found bullet holes in his uniform. But he was never wounded. During the Confederate bombardment before Pickett's Charge, General Hancock (who led one corps of the Union army) rode up and down the Union lines to exhort his men to be brave. When his aides told him to take cover, he said: "There are times when a corps commander's life does not count." He was severely wounded shortly thereafter.
So there is IRL historical precedent for Martin to be on battlefield. Whether or not in the front lines and fighting personally is a matter of opinion. IRL, in 330 BCE he would be a coward if he was not hacking bad guys with a sword. In 57 BCE he'd be a fool if he was. Of course one of the problems with Oblivion is that it is like so many Hollywood movies, which show pitched battles as these wild, confused melees with both sides all mixed up in a jambalaya stew. In reality battles were fought in dense formations of soldiers, literally standing shoulder to shoulder in a line that could go on for over a mile, with men 8-12 ranks deep. It was all about who could break the other side's line or turn their flank, becuase once an army lost its cohesion, it lost the battle. So realistically Martin could have been in the battle, but never been near the actual fighting (except of course for arrows or ranged magic). Of course the game does not give that option.
My own take on it is http://chorrol.com/forums/index.php?s=&showtopic=4431&view=findpost&p=115393 of my fan fic. I treated the Battle of Bruma as a huge affair, with an army of something like 20,000 Imperials vs. twice that in Daedra. In the Teresa fic, the Empire pulled out all the stops. The entire Fighters Guild was there. Every battlemage and healer from the Mages Guild as well. Legions were recalled from Morrowind and Skyrim to take part, their shields all painted with the black wolf of Kvatch. Plus every militiaman, mercenary, and farmer who could hold a spear was pushed into the battle. Likewise every knight in Cyrodiil, plus mercenary light horseman from Hammerfell (think Numidian/Arab Cavalry). The professional troops were put out in front. The militia, mercs and others were used as a reserve. The Fighters Guild was with the latter, as while being pros, they were not trained to fight enmasse, but as individuals. The Mages Guild likewise acted in a support role, a combination of ranged fighters and healers. The FG also had one very important task, along with the Blades. But you will have to read the chapter to find out what.
In my tale, Martin was there on the battlefield as well. But he did not direct the battle, as he was no soldier, and knew nothing about armies and warfare. He left that to Adamus Phillida. Martin was there to lend his charisma to the morale of the soldiers, and make himself a very visible figure for the troops to rally around. He wanted everyone to see that the new Emperor stood with them, and would not take a step back. Phillida himself led from the rear, and we see him actually controlling the events on the battlefield, like a master playing chess.