The user manual is not over pessimistic with that service interval. If you don't do it, the dirt builds up behind the top oil seal and grinds the stanchion's hard coating away and then rapidly eats into the softer metal underneath, ruining the fork.
Servicing is relatively easy if you can get your hands on a service kit. You need new crush washers for the compression rod bolts each time you open the fork and these are not available in the hardware/engineering trade, you have to get them from the OEM.
To service the fork you:
1) Remove it from the bike and remove all attachments such as brake calipers and mudguards.
2) You let out all air pressure (top and bottom). If you don't do this, you have a lethal weapon in your hands that will kill you, your dog or someone else.
3) You pull out the rebound adjuster at the bottom. It comes off with a forceful full and pliers may be required to grip it.
5) Now with a 5mm allen key you remove the two bolts at the bottom.
6) Now you forcefully pull off the slider unit from the crown assembly, spilling oil all over the carpet and your clothes and new lime-green Addidas.
7) Now remove the top wiper seals on the slider unit with an internally expanding seal puller or if you are skilled in such stuff, a large screwdriver.
8) Now clean out the insides of everything and get all traces of old oil out of there.
9) Disassemble the piston assembly and remove the motion control damper. Drain oil and clean. You need a circlip pliers internally expanding type. bent t nose.
9) Now replace all the O-rings, there are about 12 of them in a Reba, depending on the model.
10) Grease the piston with rubber grease and re-insert.
11) Insert the crown assembly back into the slider which by now has a new seal kit. The top wiper seal will be not be assembled but be on the stanchion.
12) Replace foot bolts and insert right amount of right oil.
13) Replace MCD and the top cap with air valve.
14) Inflate and test.
I probably took 400 shortcuts in the list above. I really don't suggest you try this by yourself at first. If you can find someone to give you a hands-on demo, that will be great. The job is dangerous, messy, tricky and easy to make you confused. Lots of wrong things can fit into wrong places and once you've disassembled it you will feel as if you have just taken a clock apart and you have too many bits and pieces to cope with.
Yet, it isn't difficult. It is just kind complex and requires some structure. I've taught probably about 500 DIYers how to service their forks and 80% of them were OK after the instruction. The rest thought it was too much bother to do by themselves or, got it wrong and came back with bundles of little plastic bags and asked us to show them again.
If you are a high-miler serious MTBer, I think you'll be better off servicing your own fork and taking the effort to learn how to do it.
