If i understand correctly then no you don't need a L3 switch, a L2 switch would do fine as long as the MPLS and internet routers could be in the same subnet on their inside interfaces ie.
R1 (MPLS) R2(internet)
192.168.5.3 192.168.5.4
L2 switch
VIP 192.168..5.1
192.168.5.5 192.168.5.6
ASA1 ASA2
apologies for poor diagram but hopefully you get the picture.
On the ASA you would then have a default-route pointing to 192.168.5.4 for the internet and then more specific routes for the remote sites via MPLS.
However you may not be able to assign both routers internal interfaces to the same subnet in which case yes, you would need a L3 switch.
Advantages/disadvantages - well with a L3 device in theory it would be possible to bypass the firewall ie. route directly between the internet and a remote MPLS site although this could be mitigated with acls on the L3 vlan interfaces.
With L2 switch you could bypass the firewall. The MPLS and internet traffic would end up on the same common vlan but this is not that unusual and they still only have the firewall to go to if they want to route anywhere else.
Obviously, whichever setup you choose you need to lock down the switch ie. restrict access to only internal use/use ssh/don't use vlan 1 etc..
Jon
