I upgraded to a VFD/Spindle on my 3 XXL last fall. Prior, I was running the Dewalt router with a SuperPID.
The router option served me well for a long time, and it’s a great option for a lot of reasons. It’s cheap, easy to get started with, and just works well for most jobs.
There were three things that drove me to the move to the VFD/Spindle.
- The cost has come down a lot
- The trim routers can have noticeable run out and play in the bearings
- The noise
Let me break those down.
Cost
This is the VFD/Spindle combo I went with. Good deal for $299 and at my house in 2 days. I went with an air cooled 1.5KW. There are beefier options out there, but this exceeds the trim routers and has more than enough power to cut anything I intended to put on my machine.
The Dewalt 611 is running about $125 on sale and SuperPID is $135 on sale, so $260. Only $40 off from buying the VFD/Spindle. Now, SuperPID is pretty amazing, it really quiets down the router gives you great speed control, including controlling speed from your gcode sender. I did a full write up on implementation here. It’s a great upgrade if you have a trim router already, it gets you really close to how a VFD/Spindle functions.
Bearings
After about 125 hours of running, I found noticeable slop in my Dewalt bearings. I did a full rebuild here, which was really cheap if you order the parts from Dewalt (surprisingly! links in post). This helped a lot, but when you see the bearings and plastic components holding things in place, you realize it’s just not meant for repeatable high precision work. Good for probably 98% of jobs people run! And they work quite well for the purposes of most people and most jobs. But it can be better. So, very inexpensive and relatively easy to rebuild, but with a Spindle you are getting all around better bearings which means less run out and movement at the collet.
Noise
Finally, the noise. I did some readings (super scientifically with my phone…) and using the SuperPID cut the router noise by half, it’s pretty amazing. The VFD/Spindle is about half of that, it is whisper quiet compared to running the router without a SuperPID. That’s a huge plus for me, with the CNC in my garage, the noise carries straight into my upstairs rooms. It doesn’t eliminate the whine of the bit cutting through some materials or shop vac or the steppers. You should still wear hearing protection! But reducing a major source of noise is a good start.
So, necessary? No, the trim routers work quite well and are very serviceable, cheap to replace or have a backup on hand. You can find factory rebuilt online for under $100. I recommend pairing with the SuperPID for the excellent speed control and quieter operation.
The VFD/Spindle was a solid and reasonably priced upgrade for my hobby. 
If you have the money, I think it’s worth it for the things I mention above. Game changer? Probably not, unless you’re making high precision parts (which a lot of people are doing with their well-maintained trim routers, because there are a lot of factors other than the router or spindle). If your current set up is working and cutting what you need and you’re happy with your workflow, stick with it.
If you’re interested, I documented my VFD/Spindle setup here. You’ll pay more for the C3D VFD/Spindle, but it is fully configured and supported. If you buy other options online, you’ll have to fiddle with it more, but lots of support and guides in the forums here.