Clearly buffing a summon without altering the duration is a buff to the summon.
I'm not against buffing summoners, or even being able to solo content if you know what you're doing. I just don't think that's the best argument to make your point.
The real issue with summons comes down to 2 things, in my opinion.
1) They soak xp
2) How strong do you make them
On point number 1, I see the merits of it, I do. However I think that soaking xp is reason enough to not ever summon them as they are. They either need to be strong enough that it's worth the xp soak or they need to not have one but be weaker - kinda like they are now I suppose. (except that they do soak xp, yannow)
On point 2, balancing summons is -hard-. There are a lot of summons and a lot of different monsters they might face and they can get a -lot- of buffs with the right groups. Sure, you can try to balance them so that they are more suitable for soloing or small groups but you can't ignore the fact that what you do to them will still be there for larger groups who might just put every spell they have on it and let it kill everything.
Do you extend the duration? Well, if you do then the buffs that you can potentially stack on them become that much more valuable. Do you reduce their duration but increase their base stats/resistances? Maybe, but then you're just nudging another piece in the diretion of power creep.
I have a hard time feeling sorry for any caster, to be honest. They are all very strong, all in their own way, if you know what their strengths are. You can definitely play a caster who likes summons. Unfortunately, people are probably not going to be happy with you for soaking up their xp and they are never going to solo everything for you. But you can use them in place of another spell to satisfy that summoning itch to almost no loss to a real groups overall ability. I don't really know what the right answer here is I'm just talking a lot.