I can see the logic in replacing certain bits IF it's cost-effective to do so on a piecemeal basis, such as Blue Hills above plans to do for a specific reason, but what I find bizarre is some people buying a certain bike then replacing half of it before a wheel has even touched tarmac. That says to me the original bike choice was poor as the maker's spec was miles away from what the rider actually wanted.
I do think that a lot of changes are not really essential though. My Raleigh Pioneer came with it's original decent quality Rigida 700c rims, but they had been bashed about during the previous 23 years and were a lot out of true. I've since rectified them to a fair degree, but not to my total satisfaction and they need more tweaking. So, to get the bike back on the road ASAP I fitted a truer set of no-name alloys off a 99p
eBay special Apollo hybrid I broke up for spare parts, and fitted brand new Schwalbes to them. I'm riding on some horribly potholed urban roads, bumpy gravel and rooty woods tracks and I'm not a lightweight rider, but the wheels are holding up fine and staying true, despite being cheaper in origin to the Rigidas. I'm not even encountering many other old-school flat bar steel hybrids on the rougher surfaces I ride on (and zero roadies on modern aero/carbon stuff), most other riders are using hardtail or even full-sus MTB's, but a relatively cheap 700c wheelset can still hold up OK if ridden sympathetically.