However, the 5 miles between home and the office has no fitness benefit at all. I just ride it and stop. At best, it maintains the ability to ride 5 miles to work.

I think, long term, it does good things for your base endurance.

If you look at my blog, you can see how few non-commute rides I do.

As others have said, I find that commuting every day ups your fitness level overall, but can knacker your weekend. Your body needs recovery time. I now tend to lay off for a few days if I'm going to do a long ride at the weekend (on or off road). I'm doing 30km per day, with about 350m of ascent.

I've found mid-week rest days really good, and stop that knackered feeling at the weekend.

Worth adding I've been doing the road to and from work for the last 6 to 8 weeks (new baby mean't not much time for anything else) so hadn't been offroad in months, but managed to do 3h at Cheddar this weekend. So I'd say it definitly can help.

Of course does nothing for downhill skills!

personally, ever since i've started commuting to work just over a year ago my fitness has increased ten fold. My daily commute is now 75 miles ( see previous post) though will be going down to 40 miles soon but i will probably add another 20 to that on the way home. My endurance levels have increased alot, also my ability to push myself hard for longer has increased as well as looking forward to long distance runs. I feel riding alot of road miles on a road bike keeps my interest high but i always make sure i drink a protein shake/ frijj milkshake afetr every run to help recovery as well as some stretching.

My downfall is probably rest days. I can quite happily go weeks, maybe a few months being on the bike every day without taking proper rest days but this is down to

A/ i'm loving riding my bike everyday
B/ i feel guilty not riding my bike on my days off and
C/ i may have an addiction to bikes just now

Keeping a mix of road and mountain biking also keeps this interest high. If i was you i'd continue biking to work and you might find yourself taking different routes home to get more ride time in. In a girly way just being on the bike and seeing whats around you is a really good feel good factor rather than making yourself go out and do miles.