Within the US, etkts are pretty much the rule these days. It would be extremely unusual to get a paper tkt anymore. You can have one, but you will pay a fee ($50 at least) to have one issued if an etkt is an option and you decline it.
When etkts first arrived it was not uncommon for problems to arise, especially when you had an itin with different carriers. Due to carriers being on different GDS systems they couldn't 'see' each others etkts. Nowadays that is rare and most carriers have agreements in place so that passengers aren't inconvenienced having to run from pillar to post trying to change tkts across carriers. This is, of course, when dealing with the major carriers. Budget carriers (Southwest, jetBlue, Spirit for example) do not share anything with anybody.
Internationally, if you are flying a major alliance carrier you will usually get an etkt. However, paper tkts are not unusual.
What I have in mind, are a couple of cases with connections on code-share flights, where the fact that I did not have a paper ticket meant that it was not possible to standby for an earlier connecting flight.
Not knowing the carriers/situation involved, I can't be positive, but saying that you can't standby for a codeshare flight because you don't have a paper tkt would be highly unusual if it happened domestically. Usually it has something to do with the fare you are on. Too, make sure that the flight you are trying to get on has a flight number corresponding to the airline you are flying (DL codeshares flights with NW/CO, make sure the NW/CO metal has a DL flight number). Airlines do not codeshare all flights, only those that (in their twisted minds) make sense, even in the same market.
Best advice is to have the record locator(s) for your itinerary. If it involves multiple airlines, you will have one for each carrier. With those, an airline can pull your etkt records for the flights that involve them. Having the ticket numbers will also help.