I never understand why many DM's feel the need to dictate arbitrary restrictions on rules as written in the core books since the core rules are the foundation for everyone's shared hopes and expectations for the game experience. The rules you use are a lot less important than recognizing the effect your players' preferences can have on the game experience.
If your players get more enjoyment from multi-classing they will be more invested in their characters and start each session from a happier baseline than if they're forced to make characters that are less than what they *perceive* as being allowed by the PHB rules. That extra investment is fuel and an opportunity for the DM to build tension and drama into the gameplay.
If the option truly leaves you feeling like you're being taken advantage of, is it because you don't think the proposed concepts fit into *your* preconceptions and personal preferences, or because you feel challenged and insecure over your ability to run a game with optimized characters? One option is you putting what you like above what your players like which can fester into personal conflicts, the other is an opportunity to build rapport with players through open discussion, maybe a middle ground compromise both sides can live with (if both sides give a little neither side feels unappreciated or disrespected), plus the second option might be something that pushes you to grow and develop into a better DM.
I have my share of house rules, but I present each one to the group, discuss why I think its an improvement and only put it into play if my players agree its an improvement for all - or at least worth testing. I also have a mix of munchkin/optimizer players and players who are happy to just be at the table with someone else doing the nuts and bolts character creation (they have a concept and someone else hands them a fitting character sheet). Indulging each player's preference give me happier players at the table leading to happier games.
If you're concerned the min/maxers will dominate play and spoil the experience for the rest of the group, share that with the group and seek buy in from the players that everyone gets a veto if things get out of control. Everyone will appreciate being considered.
That's my 2 cents worth anyway. Every group is a little bit different and YMMV.
If your players get more enjoyment from multi-classing they will be more invested in their characters and start each session from a happier baseline than if they're forced to make characters that are less than what they *perceive* as being allowed by the PHB rules. That extra investment is fuel and an opportunity for the DM to build tension and drama into the gameplay.
If the option truly leaves you feeling like you're being taken advantage of, is it because you don't think the proposed concepts fit into *your* preconceptions and personal preferences, or because you feel challenged and insecure over your ability to run a game with optimized characters? One option is you putting what you like above what your players like which can fester into personal conflicts, the other is an opportunity to build rapport with players through open discussion, maybe a middle ground compromise both sides can live with (if both sides give a little neither side feels unappreciated or disrespected), plus the second option might be something that pushes you to grow and develop into a better DM.
I have my share of house rules, but I present each one to the group, discuss why I think its an improvement and only put it into play if my players agree its an improvement for all - or at least worth testing. I also have a mix of munchkin/optimizer players and players who are happy to just be at the table with someone else doing the nuts and bolts character creation (they have a concept and someone else hands them a fitting character sheet). Indulging each player's preference give me happier players at the table leading to happier games.
If you're concerned the min/maxers will dominate play and spoil the experience for the rest of the group, share that with the group and seek buy in from the players that everyone gets a veto if things get out of control. Everyone will appreciate being considered.
That's my 2 cents worth anyway. Every group is a little bit different and YMMV.