You've never heard of the 3DS itself, then. Even an unmodified console requires the SD card to be present in order to do anything of consequence with it. That's where theme data lives, where eshop games live, where extdata such as game patches and updates live, even some cartridge games assume the presence of an SD card and save their data there rather than onboard memory.
If you wanted a piece of hardware with zero hard dependencies on removable storage, then you bought the wrong console.
I have used the 3ds and If I remove the sd card I can boot into my sysnand like its nothing. I could provide a video if you would like to prove it. If you do the samething with arm9hax you have yourself a soft brick. The fact that you need an SD card to do most things on it doesn't mean it is a necessity,I can still access my system without the SDcard. I have the o3ds and the n3ds,point is that arm9hax dramatically alters the way in which the n3ds operates. Whether you mind or not is a completely different subject,you are changing the subject to support your argument. There is no argument to be had here,take out the SD card and your 3ds is a paper weight,with menuhax I take out my SD card and I can still play 3ds games without saving(original that is).
The fact is that your 3ds is a semi-brick without the SDcard,to me that doesn't sound like a good trade off for an extra 10 seconds of booting. Your 3ds will not even turn on without an SDcard in it,to me that screams a sloppy hack. There is risk in installing arm9 and zero reward,because even if you properly install the hack you removed part of the systems features for no benefit.
At the end of the day it is personal preference and I obviously expect people that went through all the trouble of installing arm9 are going to give me a million justifications on why it is better,matter of fact is that you are handicapping your system for a minor upgrade.If I wanted to give my 3ds to a small child their is ZERO posibility of them bricking the console unusable, if you sell that arm9 console on ebay for instance the buyer can always claim you sold him a brick and at that point paypal is going to side with the buyer and not with you.It is a FACT that you removed core functionality from your 3ds and not a matter of opinion.
Anyways, I do not expect you to accept this as I know it is a matter of ego at this point and not reason. People like to argue on here even when they know they are wrong as a matter of ego,however all it takes to see the light is to remove the SDcard from your 3ds and you will see that it won't work. It is the equivalent of wearing a wig to conceal balding and calling it "regrowing hair" The truth is that underneath the wig(arm9hax) you are still bald as opposed to using rogaine/propecia where the hair is naturally yours.