It can be more than just having to show ID.
In Illinois, for example, the register jockey gets to record your Driver's License info. It might sound minor, until you get into the nasty details if Identity Theft...
http://www.highprogrammer.com/alan/numbers/dl_us_shared.html
Also, if you've forgotten your ID at home - or - *gasp* don't have an ID at all, you're SOL. Many lower-income and minority individuals do not have a state issued ID card. Isn't this the same argument used against any kind of ID laws at the poll?
Also, in Illinois, if you're pulled over and ticketed, you have the option of posting bond on-site (and, seriously, who carries $75 in cash on them anymore?) or handing over your license. Since the ticket you've been issued isn't a legal, state issued, photo ID, you won't be able to purchase the product using it (although, for those interested, you can still drive with the ticket).
Now, combine that with the fact that Sudafed has to be locked behind the pharmacy counter - meaning that if you don't live in an area with a 24 hour pharmacy, there are times when you may not be able to get what you need.
Also, the limits on what one can buy are fairly reasonable, unless you live in a rather large family and everyone gets hit around the same time.