Is it advantageous to subscribe to multiple EV charging roaming providers?

While my experience is in the US, I'd say that it's generally a good idea to have cards for where you think you'll end up needing to charge, particularly if the station is in an area with poor mobile coverage and you can't remote start with an app. Some networks have virtual RFID cards, which will be helpful where you don't have data coverage.

If you're renting a car, I'd have a plan on where to charge depending on where you think you're going to go. If you own the car, then it's a good idea to figure out places where you can routinely charge.

Yes, it is wise bring multiple cards. Sometimes cards work where apps don't, and sometimes one card works but another doesn't. My experience in Germany and The Netherlands in April 2022:

I had registered with three mobility service providers (Maingau Energie, Plugsurfing, and Shell Recharge). For the latter two, I had ordered RFID cards. For Maingau I had only the app. My experiences after charging five times, each time with a different charge point operator:

• I did not succeed in using the Maingau app. I tried it twice (using mobile data), and in both cases I got a message that I could not use their app to pay for this service provider. In one case not at all, and in one case it implied it would have worked with the card.
• In one case, the Plugsurfing card did not work, but the Shell Recharge card did. I did use the Plugsurfing card in two other cases.
• In two cases, it was possible to pay with a regular bank card, without any registration (but it might have been cheaper to use a card).
• In at least one case, where prices were clearly posted (unfortunately in April 2022 this is true for only a minority of charging locations), payment with the charge point operator (Fastned) would have been much cheaper than the cards of other charge point operators (but some charge point operators charge a monthly fee for registration, which have been the case here).