This would be ideal IMO; one family of guns. Manual of arms is the same, commonality in magazines, tools, some small parts and holsters. Some families do this better (GLOCK) than others. But you'd be able to fill multiple roles with not much variation in your inventory.
I've come to realize that I'm entrenched with Sigs and I need to let go of my other guns; but I introduce a fair amount of people to shooting(lately it has been a lot of my friends' wives & some coworkers) so having a small variety is useful for that. It also gives them options as a lot of people don't bargain shop like I do and won't go and buy a $1k new Sig...but would buy a $500 new Glock or M&P.