Weeks and Adkins suggest three arguments in favor of the integration of algebra 2/trig with geometry (although they say that many such arguments can be adduced). The arguments which they give are as follows:
1. The basic unity of mathematics and of mathematical thinking is well conveyed because the number and variety of illustrations of this unity and thinking are greatly increased.
2. The easier parts of both algebra and geometry come in the first year and the harder parts in the second year. This makes the subject easier in first year and a mature approach in the second year.
3. Both subjects are kept fresher and used in a greater variety of situations all the time so that, by the end of the second year of the program, there is firm control of all the mathematics needed for a pre-calculus course and a great feeling by the student that he knows the subject and that it makes sense to him.
Weeks-Adkins, Teachers' Manual to Second Course in Algebra with Trigonometry, pp. 3-4.