Say you run an online store. You offer USD $5 in first-time user credit.
A lot of people (even some nontechies I know) know about the "+" trick for gmail. Assuming your signup flow is easy and fast, it's very easy for those people to sign up for multiple accounts and get multiple $5 credits.
If a lot of people do that, it might significantly impact your bottom line. Not just because you have to give away a lot of inventory, but secondary effects also suck: you stop the $5-free promotional, and then all of the legitimate users who signed up during the campaign who told their friends to sign up and get some free money now have their friends bad-mouthing the site to them because "it didn't give me free shit the way you told me I should expect it to!". You might see a drop in sign-ups to below pre-promotional levels, or, worse, you might see people who signed up during the promotional trust and use your site less. I know trends/behaviors like this seem trivial--and they definitely would only affect a minority of users--but past a certain scale effects like those can have a real financial impact.
Now let's say your site is "smart" about the "+" trick and doesn't let people with gmail (or google-federated emails--boy, is figuring that out a bastard) accounts sign up multiple times. You'll lose the dubious potential business of folks who like gaming promotionals. You'll still be vulnerable to people creating second email accounts and signing up using those--but the difficulty asymmetry now favors you, the vendor: it's work for a user to make a second email account; work they probably won't do, especially if you blacklist typical temp email services like guerillamail. If the promotional is large enough to entice first-time users but small enough to deter people from doing this, you have succeeded in minimizing your loss. If the user already has a fleet of accounts for this purpose they're probably going to just take your money anyway.
Of course, there's another more annoying scenario which I'll mention because sites should never do it: sites that think they're being smart about the "+" trick by not storing the part of the address between the plus and the domain part. This is usually done to get email campaigns (read: almost entirely spam) to show up in the user's main mailbox rather than some filter-purgatory. It will drive users away in two main ways: first, if I'm technical enough to use the "+" trick and a filter to route mail, I probably have enough obsessive annoyance with spam to immediately either junk-flag yours or delete my account, compared to a small chance I would have actually read it otherwise. Second, more than half of sites that do this which I've audited parse and strip the content of the email address wrongly (parsing emails is very hard, after all) in such a way that what they end up storing could be a totally different person's email, or an invalid one. That means signups just won't work. Whatever you do, don't do this.
This doesn't only apply to first-time-user promotionals, either. It also applies to:
- Referral bonus programs.
- Services that give hand-customized products to users (think a "one per user" etsy store with one person knitting cat dolls or something): multiple similar contacts from the same user would mean you spend a lot of time making their products--time which might be wasted, even if they paid for their products, compared with time spent making them for lots of different users and increasing your recognition/exposure.
- The same applies to tech-support contact-us forms: one user can "bogart" your support staff, clogging the queue with (legitimate or not) requests and defeat your rate limiters by using the "+" trick, making other users wait a long time for replies.
- Others I haven't thought of.