The PLONK Gate constraint equation seems to designed more for accomodating adding a constant in a Gate but not multiplying with a constant

From the PLONK paper.

Page 23, 6 Constraint System

The constraint system $C = (V, Q)$ is defined as follows.

• $V$ is of the form $V = (a, b, c)$, where $a$, $b$, $c \in [m]^n$. We think of $a$, $b$, $c$ as the left, right and output sequence of $C$ respectively.

• $Q = (q_L, q_R, q_O, q_M, q_C) \in (\mathbb F^n)^5$ where we think of $q_L$, $q_R$, $q_O$, $q_M$, $q_C \in \mathbb F^n$ as "selector vectors".

We say $x \in \mathbb F^m$ satisfies $C$ if for each $i \in [n]$,

$(q_L)_i \cdot x_{a_i} + (q_R)_i \cdot x_{b_i} + (q_O)_i \cdot x_{c_i} + (q_M)_i \cdot (x_{a_i} x_{b_i}) + (q_C)_i = 0$

I understand how this equation works in cases where both $x_{a_i}$ $x_{b_i}$ are variables (i.e. intermediate variables or input variables).

However, I am little confused as to how it would work with constants when the Gate operation is multiplication.

First let's consider an addition gate which also has a constant

I want to represent

$x + 5 = var_1$

I can represent this with

$a = x$, $b = 0$, $c = var_1$
$q_L = 1$, $q_R = 0$, $q_O = -1$, $q_M = 0$, $q_C = 5$

So the equation becomes

$1\cdot x + 0\cdot 0 + (-1)\cdot var_1 + 0\cdot (x \star 0) + 5 = 0$

However, if I have a gate where there is a multiplication with a constant, e.g.

$x \star 5 = var_1$

I cannot use $q_C = 5$ to represent this gate alongside $b = 1$ & $q_R = 0$, which would be analogous (but not the same) as what we did for the addition gate.

I would instead have to set $b=5$ to represent the gate. And I don't get to use $q_C$ which is specifically for constants even if I have a constant in my gate.

Why is this designed with different treatment for adding a constant & multiplying with a constant? What this means is that I can use just a single gate for $x + var_1 + 5 = var_2$, whereas I will need to split $x\star var1 \star 5 = var_2$ into 2 gates.

The equation could have easily instead be designed as

$(q_L)_i \cdot x_{a_i} + (q_R)_i \cdot x_{b_i} + (q_O)_i \cdot x_{c_i} + (q_M)_i \cdot (x_{a_i} x_{b_i} q_{CM}) + (q_{CA})_i = 0$

i.e. the equation uses a separate $q$ for multiplication & addition, $q_{CM}$ & $q_{CA}$ respectively.

Now, I can have

$q_L = 0$, $q_R = 0$, $q_O = -1$, $q_M =1$, $q_{CM} = 5$, $q_{CA} = 0$

and

$a = x$, $b = 1$, $c = var_1$

Or another way to have similar approaches for both addition & multiplication would be to not have $q_C$ at all - neither for addition or multiplication. And use $b$ for the constant as done for multiplication with the constant.

i.e.

$(q_L)_i \cdot x_{a_i} + (q_R)_i \cdot x_{b_i} + (q_O)_i \cdot x_{c_i} + (q_M)_i \cdot (x_{a_i} x_{b_i}) = 0$

if I want to represent $x + 5 = var_1$, I can have

$q_L = x$, $q_R = 1$, $q_O = -1$, $q_M =0$$ and

$a = x$, $b = 5$, $c = var_1$

Am I missing any advantages in Plonk's approach where they adopt a different approach for adding a constant as compared to multiplying with a constant.