I would allow it. But as a conscious playstyle goal, I try not to run the kind of game in which this seems like a good idea. You can summon the monsters and you can fight them for XP-- but you only get XP for encounters if they're legitimately challenging or dangerous for you.
Since the summoner won't allow his summoned beasts to kill you, you only get half XP for the encounter if you win. If you lose, if he dismisses them, or if he orders them to pull their punches you don't even get that.
Oh? And if you're doing in this in the middle of the dungeon? Random encounter checks.
I believe in XP, and I'll let the PCs earn XP doing almost anything-- but they have to earn it.
Those restrictions sound like they would accomplish exactly zero in a group that's already prone to cheesy XP grind scenarios.
"'Legitimately challenging only?' That's fine, I'll not command my summons to stand motionless while you guys hack away - I'll summon them inside a pit instead! Now what burrowing creature can I summon to dig that pit for me..."
"Half XP only? It's OK, I'm a Focused Conjurer, I got enough spell slots to still make this work! It'll just detract twice as much from the time spent being actual heroes, but that's fine with me!"
"Random encounter? Hey, more grind, and this time, there's even loot attached! Good times! Let's stage our summons showmatches right at the next dungeon's front door, so random encounters can happen, but the way back to town is still short - that way, we can sell our phat lootz!"
If you allow summons XP grind at all, be ready to make up
a lot of rules regulating the way it works. Clever players inclined to take that route will probably still find ways to make you cry yourself to sleep after play.
To be sure, if the PCs found out they'll be going up against a powerful demon next week, and the summoner says "you know guys, let's get some practical experience fighting those things. I set up a gig and you guys try your best to hurt my champion!", that's cool with me. The PCs get a chance to find out about typical demon defenses (immunities, resistances etc.), and that's good. In-game, such an experience will probably help them in the upcoming fight. Possibly, it will even enough to make their chances a bit better mechanically - in other words, they might earn enough XP to level up before the big bad comes along (if they were close to leveling up already, that is).
However, I would grant those XP for a very good idea - not for the mitigated-risk summons showmatches themselves.
All in all, I find the idea of PCs knowing about XP and how to get them abhorrent. Furthermore, players willing to grind just to level up seem as if they're simply not interested enough in my carefully crafted plot, NPCs, world, atmosphere etc. to be worth playing with. In fact, a player suggesting to XP grind instead of exploring the scenario would feel like a slap in the face to me.