You're bound determined to justify it aren't you? What you showed is not a RJ-11 jack which is what would be used in the home. If you plug any residential phone into a jack such as you described, it will not work from the yellow/green wires. It will work from the tip/ring pair at pins 3 & 4 ONLY. You also left out pins; there are 8 on the wiring you showed, which is shown incorrectly. Telephones do not come with some wired for pins 3 & 4 and others wired for other pins: In fact, very often the silver & gray telephone cables only consist of 2 actual conductors.
If you were to try to use the yellow/black for tip & ring, you would have to terminate those wires in hte jack the phone connects to at the normal positions in the box for the red/gree pair. so the yellow wire would go where it says red and the black to where it says green. Clearly wrong.
If a telco wires in a 2-line system, they will not use 6-conductor standard phone cable but will use 8 conductor instead, and 8 pin RJ jacks vs the 6 pin RJ11 jacks. The specs dictate that so that, where the wires are no longer twisted within the jacket of the cable, a distance can be maintained between them to keep line to line crosstalk from occurring.
I detest misinformation and especially when it comes from some moron who guesses at what the rules and regs say and want to justify their own existance by giving incorrect information. Whether you looked up "tip" and "ring" or just think they were handy buzz words, they are very real and have distinct meanings within the installations. You CAN connect another line to the black/yellow pair, but then you'll have some phones that don't work and need rewiring at the box, and a very good chance of crosstalk between the two lines, especially since I doubt you would know how to keep non-twisted sections short and the rest of the things any novice installer would know. If youwant the nitty gritty it exists in the CFR, Title 47, Part 68. Go educate yourself or quit giving out misinformation that can cause other people problems.
NOW the case is closed because I've no more food for you to troll on; you're on your own.