If it ain't broke, Don't fix it!
I use my router for all the same reasons you do along with Shareport functionality.
Based on my own upgrade experience of going from 1.21NA and 0.93 Shareport, to 1.33NA and 1.10 R Shareport on A4 version of 655.......I am very thankful of having the ability to downgrade provided by another poster.
1.21 works fine for me, having figured out what settings keep my router stable, including Shareport ver. 0.93 which has no difficulty mounting as many as 4 different external drives and or printers (one at a time) attached via powered USB hub, from any computer on my network with Shareport installed.
Firmware 1.33NA seemed to offer some further advantages in security and setting's flexibility, so I upgraded.
The firmware itself seemed to be stable....I have never run it for more than a day.....my problem was with a non functional Shareport ver 1.10 R, in regard to my external hard drives. It was just plainly wonky!
It would see the drive, mount OK, but when I selected a file, it would loose connection and hang Win Explorer or whatever app I was using to open the file, until I reconnected the drive via Shareport utility and round and round it would go.
When this would happen, I would also drop my Internet connection?
If I did not use Shareport, all seemed fine. In fact, that version of Shareport did work fine with my printers....just not my hard drives.
I did a hard reset to my router prior to upgrade before, as well as after, and manually entered my settings. I had previously uninstalled the earlier version of Shareport and installed the 1.10 R version with changes in my Win Firewall Advanced settings and within Shareport itself as to broadcast addresses. I reflashed my firmware to make sure I did not have a bad flash and upgraded/downgraded several times over the course of a week.
None of my efforts were productive in regards to shareport and its conflicts with my hard drives on any computer within my network.
Again, I must state Shareport ver 1.10 R worked fine with my printers.
Whatever I would gain with 1.33 NA firmware did not balance against my Shareport problems with my hard drives, especially when all worked well with original firmware/Sharerport.
So after each day of frustration, I would downgrade back to 1.21 & original Shareport, and all would be good again. I have upgraded/downgraded 3 times in my efforts to get Shareport working properly (firewalls on/off, firewall settings changed, re-flashes of firmware)...enough already!
So, YMMV....If you do not use Shareport or use it just for printers, you may be fine with an upgrade.
However, if you are not educated in the use/settings of your router, and it works just fine for you as it is....Don't fix it...... Personally, I would not advise it.
As far as I could tell (please enlighten me if I am incorrect) "the HNAP security issue that 1.33NA fixes, isn't that serious, it is only an inside-LAN-issue" and no D-Link representative has officially stated it was an issue with the 1.21 or lower firmwares.
So unless you have some malevolent individual who is ethernet cabled to your network, this is a non issue.
Thank you for the time you took to read this, hopefully this will save you hours of unwarranted frustration if you have a similar experience such as I did.