Well, yes, but that wouldn't possible if, as some other poster said, when you go for one role, you get restrictions for the other. If you can't get skills of all sorts, because one prohibits the other, it's going to be difficult to go for the role of 'jack of all trades', and even a more complex roleplaying where different types are combined, depending on the time of day. It also would make things difficult in those roles, if, as another person suggested, getting into one guild would prohibit getting in another. So, if I'm a member of the mages guild, I can't be in the fighters guild, and/or in the thieves guild and/or the dark brotherhood. Well, that's a partypooper for playing a role where you play a battlemage at day and a murderous thief at night.
Being able to mix your skills and thus, the ability of being a jack of all trades was exactly what other posters were complaining about.
I think this is a non-problem.
TES has always allowed us to create our own characters, with their own skillsets.
As for the "battlemage by day and thief by night", it's a fully viable build(although I cannot imagine you getting much sleep), but that build will(should) force you to compromise. A jack of all trades will never be as good as a "true" role, but will have versatility.
And when that jack of all trades starts to mingle with the guilds, membership in one guild shouldn't prohibit another, but it will(should) be harder. I want to see requirements back for the guilds. The fact a half-brained orc barbarian, with no knowledge of anything but smashing stuff up, could rise to the top of the mages', thieves' and assassins's guild in OB without breaking anything but a light sweat and a handful of lockpicks makes me wanna cry.