The way these rules look to me, a sniper/medic would be better executed with a TAC or Dev marine with medicae as an elite skill and requisitioning a narthecium, without house ruling something.
from a rules crunch aspect, you are undeniably correct.
however, there are a number of fluff and character development points that you are ignoring. While an apothecary trained in medicae is mechanically not all that different from any other marine trained in medicae, the implied level of training is completely different. your standard Space marine probably knows basic first aid and has enough knowledge of Astartes physiology to staunch a bleeding wound or stabilize an ally, but he's no doctor. Apothecaries, on the other hand, are trained specifically to tend to the wounds of space marines, as well as the many intricacies of the Astartes Body, therefore making them better trained medics. I don't have a book in front of me, but if i remember correctly, this is exemplified by the Apothecary's ability to quickly gain advancements in the Medicae Skill and a slew of Medicae related talents that can only be taken by other Marines through elite advances. Furthermore, it is my understanding that Deathwatch Apothecaries recieve further training on Xenos Physiology upon taking the Apocryphon Oath and are charged with learning even more about the Xenos body in the execution of their missions, so as to learn how to better combat the xenos and exploit inherent weaknesses. This puts the Apothecary in a unique position to gain unparalleled knowledge of what will hurt the likes of Tyrannid and Tau the most, and it is An Apothecary who will often find it easiest to exploit that knowledge. Also, Space Marines aren't as Flexible as some people believe them to be. just because an Assault Marine CAN learn the Medicae skill, or a Tactical Marine CAN requisition a Narthecium or even make it standard issue through Signature Wargear, that doesn't mean he would. for what reason would anyone but an Apothecary have a Narthecium? Finally, Removing the Progenoid Glands, which is ultimately an Apothecary's chief duty, is a delicate process for which Apothecaries are specifically selected and trained. For this reason alone, Apothecaries are not only useful, but indeed necessary for a Kill Team. Who but the Apothecary would know how to harvest the sacred Geneseed from a fallen Brother? who would teach anyone but an apothecary to harvest the Geneseed? and why would anyone but the Apothecary want to know in the first place?